J . Phys. Chem. 1986, 90, 6091-6101 cobalt-oxygen superoxide adducts characterized by EPR and IR spectra similar to those obtained in the case of cobalt homogeneous oxygen carriers. On the basis of the results obtained, the possible existence of the following surface species was proposed: (1) bent “end-on” Co3+-02- species I, I1 (structures i and/or ii) with no interaction with adjacent ions; ( 2 ) bridged Co3+-O~-Mg2+species (structures iii and/or iv) involving cations on two vicinal planes, and “end-on” Co3+-Oc adducts stabilized by interaction with surface Mg2+cations located on the same plane (species 111); (3) bent “end-on” adducts interacting with adjacent surface hydroxyl groups (structures v and/or vi); (4) “side-on” superoxide species stabilized on Mg2+ ions (species IV). The two-step interaction of oxygen with the COO-MgO surface was put into evidence: the first stage, where activation of dioxygen occurs giving rise to species I and 11, is followed by the next stabilization step. This second stage is achieved upon warming the samples to 120-140 K and the initially activated superoxide
radicals (I and 11) undergo stabilization due to interaction with Mg2+ions (species 111) or are isolated on the MgO matrix and stabilized on surface magnesium cations (species IV). The whole process described in this paper represents an original pathway of the dioxygen activation on oxide surfaces. A fraction of cobalt ions which are able to bind oxygen reversibly may serve as an example of heterogeneous analogues of homogeneous oxygen carriers.
Acknowledgment. E. Giamello acknowledges the receipt of a NATO collaborative research grant (No. RG 86/0556). M. Che dedicates the work he carried out in this study to the memory of Juri Kukk, Estonian Professor of Chemistry, who died in a Soviet Labor Camp on March 27, 1981 at the age of 40 and also to Y. Tarnopolsky. Registry No. 02,7782-44-7; OF, 11062-77-4; COO,1307-96-6; MgO, 1309-48-4.
A Theoretical and Experimental Investigation of the Dication of Nitric Oxide R. W. Wetmore? and R. K. Boyd*$ Guelph- Waterloo Centre for Graduate Work in Chemistry, University of Waterloo, Waterloo, Ontario, Canada N2L 3G1, and University of Guelph, Guelph, Ontario, Canada N1 G 2 W l (Received: February 20, 1986) The doublet and quartet states of the diatomic dication NO2+have been investigated by ab initio CI calculations. Energies and tunnelling lifetimes of all rotational-vibrational levels, of the two quasi-bound states thus characterized,have been calculated. Some new experimental results are presented, and an extensive discussion is given of known theoretical results vis-54s experimental information available for NO2+. The latter include data from electron-impactionization onsets, Auger spectroscopy, double-charge-transfer spectroscopy, translational energy loss spectroscopy, and measurements of kinetic energy release accompanying predissociation over a wide range of time scales.
I. Introduction Optical spectroscopy has led to a wealth of detailed knowledge concerning diatomic species.’S2 One class of diatomic molecules about which relatively little is known, however, is represented by the dications XY2+. The existence of such species which are stable for at least a few microseconds, despite the Coulombic repulsion between the constituent atomic ions, has been established for more than 50 years3 However, the difficulty in maintaining suitably large concentrations for sufficient periods of time has prevented application of techniques of high-resolution optical spectroscopy to studies of XYz+. Until recently, the only such study was of an emission band attributedSto an electronic transition of N22+. Modem techniques using laser photofragment spectroscopy of fast (keV) ion beams have since been applied6 in an elegant study of this system. However, in no other case is information available from high-resolution optical spectroscopy for diatomic dications. Understanding of these species must currently be sought through less direct methods. The potential importance of these species in the ionosphere has been emphasized.’ In fact XY2+ ions can be formed in surprisingly high yields whenever a diatomic precursor XY is exposed to an ionizing agent of sufficiently high energy. At least some states of the XY2+ ions are sufficiently stable that they may be collisionally thermalized. When they do dissociate, however, the X+ plus Y+ fragments are in general highly nonthermal, with excess energies per fragment of typically 2-3 eV or more. Thus, the subsequent effects of formation of XY2+could be substantially more important than the primary yields might suggest. Experimental investigations of XY2+species have been carried out using methods which include Auger spectroscopy, doublecharge-transfer spectroscopy, and various forms of translational t University
of Waterloo. *University of Guelph.
0022-3654/86/2090-6091$01.50/0
spectroscopy which measure kinetic energy release upon unimolecular and collision-induced dissociation. Correlation and interpretation of these disparate sources of information is greatly enhanced by the global perspective provided by accurate a priori calculations of potential curves for the species. The present work on NO2+represents one in a series of theoretical investigations of diatomic dications. Previous species thus studied*-’O have been CH2+, C 0 2 + , and N22+. Theoretical investigations of the bonding effects in dications XY2+have evolved along two independent paths. An early discussion of Hez2+by Pauling” employed the concept of resonance between valence-bond structures He+-He+ and He2+-He. The former structure contributes an essentially repulsive contribution, but the latter can give rise to bonding. This same physical picture can be identified in subsequent theoretical treatments of Hez2+ using the molecular orbital approach,I2J3where the bonding interaction may be ascribed to polarization of electrons from the neutral He, by the charged a particle at long range, and by bonding into the available molecular orbitals at shorter distances. (1) Herzberg, G. Spectra of Diafomic Molecules, 2nd ed.; Van Nostrand: Princeton, NJ, 1950. Herzberg, G . Spectroscopic Constants of Diatomic (2) Huber, K. 0.; Molecules; Van Nostrand: Princeton, NJ, 1979. (3) Friedlander, E.; Kallman, H.; Lasereff, W.; Rosen, B. Z . Phys. 1932, 76, 60.
(4) Carroll, P. K. Can. J . Phys. 1958, 36, 1585. (5) Carroll, P. K.; Hurley, A. C. J . Chem. Phys. 1961, 35, 2247. (6) Cosby, P. C.; Moller, R.; Helm, H. Phys. Rev. A 1983, 28, 766. (7) Prasad, S. S.; Furman, D. R. J . Geophys. Res. 1975, 80, 1360. (8) Wetmore, R. W.; Boyd, R. K.;Le Roy, R. J. Chem. Phys. 1984,89, 329. (9) Wetmore, R. W.; Le Roy, R. J.; Wetmore, R. W. J . Phys. Chem. 1984, 88, 6318. (10) Wetmore, R. W.; Boyd, R. K. J . Phys. Chem., in press. (11) Pauling, L. J . Chem. Phys. 1933, I , 56. (12) Browne, J. C. J . Chem. Phys. 1965, 42, 1428. (13) Yagisawa, H.; Sato, H.; Watanabe, T. Phys. Rev. A 1977, 16, 1352.
0 1986 American Chemical Society
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The Journal of Physical Chemistry, Vol. 90, No. 23, 1986
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the HONDO program package.20 The molecular orbitals thus generated were used for all subsequent C I calculations. The atomic basis sets used were the Gaussian set of Huzinagaz’ as contracted by D ~ n n i n g ,augmented ~ ~ , ~ ~ with a set of Cartesian “d” polarization functions composed of a two-component fit to molecular states E, eV a Slater orbital of exponents 2.21(N) and 2.28(0). This yields 0.000’ a basis of essentially double-{-plps quality (9s5p2d/3s2pld). The 1.899 S C F molecular orbitals, thus obtained, were partitioned into three 3.324 sets for the subsequent CI calculation: 4.053 1. frozen orbitals (1-20, 13-140), of fixed occupation in all 5.018 configuration state functions (SCF’s); 5.223 2. active space (3-60; 1-2n); 6.9 17 3. virtual space (7-120; 3-6a; 1-26). 7.374 9.071 Two levels of CI were used. A first-order CI (FOCI) basis was created by considering all single excitations to the virtual space from the multireference set of configurations composing the complete active space, and resulted in configuration spaces ranging Only recently has H q 2 +been observed by using an ingenious mass from 100 to 500 CSF‘s. The second C I basis consisted of a spectrometric technique.I4 The electron polarization effect, resingles and doubles C I (MR-SDCI), formed as above, ferred to above, was also proposed by Dorman and M o r r i ~ o n ~ ~ ~ multiroot ’~ but with single and double excitation from the CAS. This second to account qualitatively for the stability of several diatomic dibasis resulted in configuration spaces in the range 3000-5000 cations observed by them in mass spectrometric experiments. In typical cases the FOCI will be qualitatively correct, CSF‘s. An alternative approach due to H ~ r l e y proposed ’ ~ ~ ~ ~conbut with a nearly constant shift in energies from the more extensive struction of potential curves for XYz+ as superpositions of CouSDCI. Cases where this is not so serve as check or warning to lombic repulsive curves upon “normal” curves, obtained by scaling quickly identify potential problem spots in the accurate surfaces. (via the virial theorem) of spectroscopically known potential curves Only final accurate SDCI values are reported below. for the isoelectronic neutral species (e.g., C N for NO2+). This One calculation was performed for each set of states of a given approach has been used widely in the interpretation of experispacespin symmetry within the C,, point group, using a program mental data of various kinds. However, it is inherently incapable based on the method of Segal and W e t m ~ r e . ~In~this , ~ ~approach, of accounting for charge-polarization contributions discussed calculations are carried out in a limited configuration space or above, since no analogous interaction is present in the case of a “core Hamiltonian”, selected on the basis of the states of interest neutral molecule like CN. However, this a p p r ~ a c h ” , ’is~ an and the strength of interactions with this initial “nucleus” of CSF‘s. ingenious method for extraction of the bonding components. The individually small but cumulatively significant interactions In ab initio calculations on XY2+ which account for all apof the remaining configuration space are included through a propriate configurations, all such effects would be automatically perturbative “effective Hamiltonian” c o r r e c t i ~ n .It~ is ~ ~well~~ included. The only serious calculation of this nature for NO2+ known that, in such methods, the choice of the “nucleus” conin the literature to date is due to Thulstrup et al.I9 These authors figurations can have significant effects on the accuracy of the states employed full valence configuration interaction using a minimal generated from a single calculation. However, given a choice of STO basis set to compute potential curves for several doublet nucleus which adequately represents the physical problem, the states. Such a level of calculation is expected to yield potential stability of the method is good to within energies of the order of curves that are qualitatively correct but of low accuracy. (In the tenths of an electronvolt. As has been discussed previouslyt0 in present context, the lack of flexibility in the basis to account for some detail, in the present context of calculations on XY2+species, polarization effects could cause significant difficulties.) Recent the method is particularly reliable for the lower states of each advances in post-Hartree-Fock techniques now permit full CI spin-space symmetry (stable to within 0.1 eV). However, it treatments with a large basis set, using (by modern standards) becomes progressively more difficult to achieve an equal balance modest computational resources. The approach adopted here for in the nucleus for the upper states. NO2+ was the same as that employed previouslyE-’Ofor other Almost all states of XY2+species of finite lifetime, characterized systems, viz., extraction of physically significant aspects of the thus far, are quasi-bound in nature; Le., the u = 0 vibrational level calculation for a full treatment, with the remaining terms taken lies at an energy above that of the adiabatic dissociation limit for into account as an effective potential by using perturbation theory. that state. In order to calculate the energies and widths (lifetimes) Details of this procedure are given below. of the rotational-vibrational levels of such quasi-bound states, the method used in the present work was that developed by Le Roy 11. Theoretical Approach and B e r n ~ t e i n . ~ ~This . ~ ’ method is one of the most efficient and a. Theoretical Procedures. Potential surfaces were obtained reliable of such procedures and is available as the computer for the low-lying doublet and quartet states of various symmetries, program LEVEL.^^ 2-4[ Z+,Z-,II,A,@], correlating with asymptotic limits for dissociated atomic ion states up to 10 eV above the lowest such limit. These (20) Dupuis, M.; Rys, J.; King, H. F. QCPE 1981, 13, 403. states correlate with asymptotic limits corresponding to atomic (21) Huzinaga, S . J. Chem. Phys. 1965,42, 1293. ID, IS)and to atomic oxygen ions in their s2p3configurations (3P, (22) Dunning Jr., T. H. J. Chem. Phys. 1971, 55, 3958. nitrogen ions in their szp2 configuration (3P, ID, IS). These (23) Dunning Jr., T. H.; Hay, P. H. In Modern Theoretical Chemistry; asymptotic correlations are summarized in Table I. Schaefer 111, H. F., Ed.; Plenum: New York, 1977; Vol. 3. (24) Segal, G. A.; Wetmore, R. W.; Wolf, K. Chem. Phys. 1978, 30, 269. The a b initio calculations were conducted for internuclear (25) Diamond, J. J.; Segal, G. A.; Wetmore, R. W. J . Phys. Chem. 1984, separations covering the range from approximately 1-5 A. Re88, 3532. stricted Hartree-Fock S C F calculations were conducted on the (26) Le Roy, R. J.; Bernstein, R. B. J. Chem. Phys. 1971, 54, 5114. singlet ground state of the nitric oxide monocation NO+, using (27) Le Roy, R. J.; Liu, W.-K. J. Chem. Phys. 1978, 69, 3922. TABLE I: Asymptotic State Correlations and Experimental Energies for NOZ+ atomic limits N+ O+
(14) Guilhaus, M.; Brenton, A. G.; Beynon, J. H.;RabrenoviE, M.; P. von R. Scheleyer, P. von R. J. Phys. B, 1984, 17, L605. (15) Dorman, F. H.; Morrison, J. D. J. Chem. Phys. 1961, 35, 575. (16) Dorman, F. H.; Morrison, J. D. J . Chem. Phys. 1963, 39, 1906. (17) Hurley, A. C.; Maslen, V. W. J. Chem. Phys. 1961, 34, 1919. (18) Hurley, A. C.; J . Mol. Spectrosc. 1962, 9, 18. (19) Thulstrup, P. W.; Thulstrup, E. W.; Andersen, A,; Ohm, Y. J . Chem. Phys. 1974, 60, 3975.
(28) Le Roy, R. J. University of Waterloo Chemical Physics Research Report CP-230R, University of Waterloo, Waterloo, Ontario, Canada. (29) OKeefe, A.; Illies, A. J.; Gilbert, J. R.; Bowers, M. T. Chem. Phys. 1983, 82, 47 1. (30) Jonathan, P.; Boyd, R. K.; Brenton, A. G.; Beynon, J. H. Int. J. Muss Spectrom. Ion Processes 1986, 68, 91. (31) Appell, J.; Durup, J.; Fehsenfeld, F. C.; Fournier, P. J . Phys. B 1973, 6, 197. (32) Moddeman, W. E.; Carlson, T. A.; Krause, M. 0.;Pullen, B. P.; Bull, W. E.; Schweitzer, G. K. J. Chem. Phys. 1971, 55, 2317.
The Journal of Physical Chemistry, Vol. 90, No. 23, 1986 6093
The Dication of Nitric Oxide
TABLE 11: Computed Energies (eV relative to Y = 0 Level of X211 State of NO) for Selected States of NOztu Doublet States R,8, X22+ 2z+ 2z28A2n 2n 0.750 0.900 1.000 1.064 1.120 1.200 1.350 1.500 1.650 1.800 2.000 2.200 2.500 4.000
60.741 43.108 39.880 39.425 39.585 40.1 15 41.520 42.499 43.170 42.730 41.552 40.553 39.703 37.127
84.689 59.138 52.065 49.607 48.270 47.157 46.276 45.776 45.283 44.833 44.191 43.598 42.807 40.634
84.095 59.165 52.354 50.095 48.943 47.956 46.920 46.416 46.079 45.466 44.599 43.720 42.798 40.634
87.486 61.929 54.856 52.425 5 1.082 49.836 48.410 47.728 47.223 46.636 45.923 45.357 44.773 42.728
69.837 48.141 42.666 41.015 40.334 39.975 40.258 40.818 41.341 41.323 40.844 40.230 39.439 37.186
8 1.924 59.563 53.529 51.261 49.218 47.173 44.869 44.043 43.952 43.928 43.805 43.524 42.885 40.582
2A
2@
84.296 59.428 52.584 50.314 49.1 18 48.141 47.262 46.674 46.211 45.433 44.439 43.754 42.853 40.648
95.163 65.580 56.189 52.514 50.221 47.999 45.784 44.779 44.465 44.121 43.693 43.256 42.644 40.63 1
Quartet States
R,8,
48+
42+
411
0.750 0.900 1 .ooo 1.064 1.120 1.200 1.350 1.500 1.650 1.800 2.000 2.200 2.500 4.000
80.276 55.347 48.657 46.47 1 45.338 44.462 43.739 43.560 43.277 42.439 41.261 40.335 39.362 36.974
83.126 58.276 51.552 49.307 48.062 46.967 45.960 45.343 44.584 43.628 42.539 41.771 41.017 39.041
92.308 62.819 53.509 49.889 47.649 45.489 43.491 42.519 42.022 41.440 40.761 39.935 39.208 36.928
4A 81.777 56.919 50.221 48.020 46.832 45.864 45.094 44.794 44.598 44.227 43.315 42.423 41.485 39.099
4@
114.705 92.457 81.384 75.463 70.617 64.777 56.833 51.886 48.761 46.654 44.859 43.753 42.773 40.621
'The u = 0 level of the X22+ state was assigned an energy of 39.5 eV (see text). The calculated value, relative to all particles at infinity, was -128.027 35 atomic units.
b. Theoretical Results. In all, potential curves for 66 doublet and quartet states of NO2+were thus calculated. The calculated potential surfaces for the lower states of each symmetry are listed in Table 11, and plotted in Figures 1-4. The qualitative features of such curves for diatomic dications have been discussed previously8-'o at length and need only be mentioned briefly here. The unusual shapes often exhibited by these adiabatic potential curves are due to their constitution from at least three diabatic states of the same space-spin symmetry, viz., an essentially Coulombic repulsive curve plus two bound states which differ only in orientation (u or T ) of a pair of bonding electrons, and which correlate with X2+ Y (e.g., N2+plus 0 in the present case). When the orbital energies are near-degenerate, overlap considerations make the u2-state the preferred occupation at larger internuclear distances, while the T2-state is more stable at shorter distances.8-10 In such cases these two diabatic curves must cross each other at intermediate values. In addition, each is crossed by the diabatic repulsive curve correlating with X+ plus Y+.Depending on the dispositions of these three diabatic states, the adiabatic surfaces formed from them under the auspices of the noncrossing rule will display knees, shoulders, inflections, humps, etc. The multipartite character of these states requires careful balancing of orbital and correlation effects, and is often only clearly shown when a manifold of curves is plotted by using a fine mesh and the diabatic trends interpolated from a number of adjacent states. The present calculations show that only two states of NO2+are quasibound, viz., the X22+and A211states. (This ordering of the states agrees with that obtained by Hurley,18 but is the opposite to that proposed by Thulstrup et a1.19). The computed energies and tunnelling lifetimes of the vibrational levels of these two states, for zero angular momentum associated with molecular rotation,
+
(33) Kim,Y.B.;Stephan, K.; Mark, E.; Mark, T. D. J . Chem. Phys. 1981, 74, 6771. (34) Newton, A. S . ; Sciamanna, A. F. J . Chem. Phys. 1969, 50, 4868. (35) Beynon, J. H.; Caprioli, R. M.; Richardson, J. W. J. Am. Chem. Soc. 1971, 93, 1852.
TABLE III: Vibrational Levels of the Two Quasi-Bound States of NO2+" X22+ (0; 39.568; 78.4) (1; 39.839; 67.0) (2; 40.091; 57.6) (3; 40.338; 49.5) (6; 41.049; 29.7) (9; 41.701; 14.0) (12; 42.260; 2.0) (15; 42.747; -7.0) A211 (0; 40.046; 28.5) (3; 40.552; 10.9) (6; 40.936; -1.7) (9; 41.310; -11.3)
(4; 40.579; 42.3) (7; 41.275; 24.0) (10; 41.898; 9.6) (13; 42.428; -1.3) (16; 42.899; -9.4) (1; 40.214; 21.9) (4; 40.664; 6.3) (7; 41.067; -5.3)
(5; 40.817; 35.7) (8; 41.493; 18.8) (11; 42.084; 5.6) (14; 42.590; -4.3) (17; 43.042; -11.6) (2, 40.373; 16.0) ( 5 ; 40.802; 2.1) (8; 41.193; -8.5)
"For each level, the vibrational quantum number u, energy (eV) above the u = 0 level of ground-state NO (X211),and log (tunnelling lifetime in s) are given for zero angular momentum of molecular rotation.
are given in Table 111. Reference will be made below to energies and lifetimes of higher rotational levels, as the requirement arises. In view of the importance of these two states in the subsequent discussion, it must be emphasized that the two curves cross at an internuclear distance of 1.2 A (Figures 3 and 4). This corresponds closely to the u = 2 level of the X2Z+ state, and to an energy between the u = 0 and u = 1 levels of the A211. The dipole radiation selection rules are such that radiative transitions between all levels these two states are fully allowed. Thus a t R > 1.2 %., of the X2B+ state radiate from their outer turnin points to the A211state, on a time scale s. For R < 1.2 , on the other hand, all levels of the A-doubled A211 state will radiate to the X28+ state from their inner turning points.' However, the u = 0 level of the AZIIstate (40.046 eV, Table 111) lies just below the crossing with the X2Zt state and thus is expected to radiate with very low efficiency. These conclusions have implications for identification of the states of NO2+which can survive for a few microseconds in a mass spectrometer or similar apparatus. Any NO2+ions, formed initially in the X2Z+ state by electron impact ionization within the
-
w
6094
The Journal of Physical Chemistry, Vol. 90, No. 23, 1986
Wetmore and Boyd
2 Glgma +
NO++
NO++
2 81"
-
2.0
2.6
0
i m > O
o i C
- 0
Lur3' 0 0 1
1
I
I
1.0
R
1.6
In
1
I
2.0
2.6
J 1.0
NO++
1.6
R In Ang8trOm8
Angrtroml
NO++
2 P1
2
Delta
0
i > - i Q c '
0
U l i 0
0 0 1
1
I
1.0
1.1
1
2.0
1
1
2.1
R In Angstrom8
1.0
1.6
2.0
2.6
R I n Ang8trOm8
Figure 1. Doublet states for NOZ+,separated according to space symmetry.
Franck-Condon region ( 1.117-1.185 A, defined by the v = 0 level of the X211 ground state of NO), will rapidly (- 10+ s) relax to the u = 0, 1, or 2 levels by radiative relaxation via the AZIIstate. Since the u = 0 level of the A211state lies just below the crossing with the X2X+ state, it also may be populated by these processes and survive for a few microseconds. Those NO2+ ions which are formed initially in the A211 and other excited states are lost via either dissociation or radiative relaxation to the low quasi-bound levels of the X22+ and A211 states, referred to above.
111. Experimental Investigation a. Experimental Method. Some experiments were repeated in the present work with a VG Analytical ZAB-2F mass spectrometer. In this instrument the ions are formed in a conventional electron impact ionization source and were accelerated through 8 kV, and the NO2+ ions were selected according to their m / z values by appropriate setting of the sector magnetic field. The selected ion beam is then transmitted to a very long (1 m) field-free region with a differentially pumped collision gas cell located near the focal point of the magnet. The indicated background pressure mbar on the ion gauge located near in this region was 6 X the collision cell. Ion translational energy spectra were obtained by scanning the voltage supplied to the cylindrical electric sector. Multiple-scan accumulation was achieved by using a Tracor Northern Model 1550 digitizer and signal averager. b. Experimental Results. Figure 5 shows an overview of the translational energy spectrum of 0" ions formed from NO2+ ions accelerated through 8 kV and in collision with helium. The
four major features of this spectrum are as follows: (i) An intense central feature arising from a sequence involving electron transfer
NOo2+2NO+(+He'+)
-
O'+(+N')
(ii) An interior pair of peaks, disposed symmetrically about feature (i), which arise from direct dissociation of NOo2+to N+ plus 0". These two peaks are the extremities of a single peak due to one such process, the central portion of the peak having been lost due to severe discrimination against 0" ions formed with large velocity components along the slit-height direction (zero electrical focusing). This internal pair of peaks corresponds to a translational energy release of 6.1 eV, if the peak width is measured between the maxima. However, the peak sides are sufficiently oblique that it is clear that a range of values of translational energy release is involved in the collision-induced process. (iii) A similar external pair of peaks, which correspond to a translational energy release of 8.1 eV. Similar remarks apply as in (ii). (iv) An extremely broad feature extending out to values of translational energy release at least as high as 15 eV. This broad feature underlies the external peaks described in (iii) above, but interferes much less seriously with the internal pair described in (ii). Exactly analogous features are observed in the corresponding N+ spectrum, but this is not shown here. As the electron energy
The Journal of Physical Chemistry, Vol. 90, No. 23, 1986 6095
The Dication of Nitric Oxide
4 Slgmr
NO++
+
NO++
-
4 Slgna
0
:
0
4 1
1
I
I
1.0
1.5
R
ln
I
I
2.0
I
2.6
I
R
Ang81rOm8
NO++
I
1.0
I
1.6
In
I
2.5
Ang81rOn8
NO++
4 PI
2.0
4 Delta
0
q
1. 0
R
t.6
2.0
1 1.0
2.6
R
In Angrlronr
1.1
2.0
2.5
In Angrtron8
Figure 2. Quartet states for NO2+,separated according to space symmetry.
NO++ K e y S t a t e s
I
I 0.8
1.2
1.5
1.8
2.1
R In Angstroms Figure 3. Ten key states for NOZ+.
2.4
2.7
is decreased below about 60 eV, the relative intensity of the broad feature (iv) falls off significantly. Within the rather large uncertainties it was not possible to distinguish between the appearance energies of (ii) and (iii) (40 f 5 eV). It was important to determine whether or not a truly unimolecular dissociation of NO2+could be observed. Figure 6 shows partial ion kinetic energy spectra, for both the o'+and N+ product channels, for dissociation of NOZ+in the absence of added collision gas. The process (i), dissociative electron capture from the background gas in the vacuum system, clearly has an extremely large cross section. The N + channel, since it appears at kinetic energies below that of the main beam of precursor ions, is subject to artifact peaks of various kinds. For present purposes, it is significant that the process (ii), characterized by the lower value of translational energy release, appears to survive in the absence of added collision gas, and it thus was important that both product channels be observed. The features with the larger values of translational energy release do not survive at the lowest available pressure (Figure 6 ) . Thus, under experimental conditions such that dissociative electron capture (peak (i)) was still observed (Le., a process requiring a collision), process (ii) was observed but (iii) was not. Figure 7 shows the results of a careful study of the dependences of these processes, in the o'+channel only, upon the pressure of helium as collision gas. While there was some uncertainty as to how to deconvolute the overlapping peaks, the most important qualitative conclusions to be drawn from features of Figure 7 are independent of the assumptions made in this regard.
6096
The Journal of Physical Chemistry, Vol. 90, No. 23, 1986
Wetmore and Boyd
NO++ K e y S t a t e s 0
N (I)
0
0 (I)
0
>,
W g m u d Pressure = 6x10.' mbar
i t 0
= d
- t
t
1
I 1.0
1.2
1.0
1.4
1.8
2.0
2.2
R I n Angstroms
-
Figure 4. Seven key states for NOZt plotted on a large scale.
n N$+
--+ 0'
Figure 6. Partial ion kinetic energy spectra, both 0" and Nt product channels, for dissociation of NOZtin the absence of added collision gas. Each spectrum represents the accumulation of 400 scans and shows peak (i) (dissociative electron capture) together with one half of the peak for process (ii) (T = 6.1 eV). The Nt channel is subject to artifacts, e.g., the intense sharp feature corresponds to NO+ 0" (+N) occurring during acceleration from the ion source.
[ + N+ ]
He CID
P(mbar x 109) i
1.00
1.05
1.10
l
€4
Figure 5. Translational energy spectrum of 0" ions formed from NUZt ions via collisions with helium (indicated pressure 2 X lo-' mbar). Electron energy 100 eV, accelerating potential 8 kV. The off-scale feature is the main beam of NO'*+ precursor ions.
Figure 8 shows the information contained in Figure 7, plotted as intensity vs. pressure of collision gas. Thus, process (iii) is reduced to unobservable levels (Figures 7 and 8) in the absence of helium collision gas; i.e., the cross section for process (iii), in collisions with the background gas, are much smaller than with helium. The experimental points for process (i) can be extrapolated to the origin, suggesting that cross sections for dissociative electron capture by NO2+,in collisions with the background gas and with helium, have similar values. The main interest, however, centers on process (ii) (that with the smaller value (-6 eV) of the kinetic energy release). The crucial question concerns the appropriate extrapolation of the corresponding line (Figure 8) to zero pressure. Simple linear extrapolation of the experimental points predicts zero intensity for process (ii) at an indicated pressure of 4 X lv Torr, but such a procedure is difficult to justify on physical grounds. The dashed-line extrapolation in Figure 8 was drawn on the assumption that process (ii) is a purely collision-induced process, (Le., the extrapolation must include the origin). The result of this assumption implies that the background gas (mostly CO and H20, with some aromatic residues from diffusion-pump fluid), must be much less efficient than helium in inducing process (ii), and almost wholly ineffective in inducing process (iii). This conclusion is in qualitative agreement with results obtained previously36 by (36) Curtis, J. M.; Boyd, R. K.J . Chem. Phys. 1984, 81, 2991.
CID He t backgmpd
", 0'
II
(t Nt)
)
Figure 7. Partial ion kinetic energy spectra (0"channel only) for dissociation of NO2+at various pressures of helium added as collision gas. Indicated pressures are those read on an ion gauge located between the collision cell and the diffusion pump nearby and are thus only proportional to pressures within the cell.
using Nz, argon, or NO itself as collision gases, and extended to the case of CO in the present work. Thus, the combined evidence supports the view that process (ii) can indeed be induced, with low efficiency, by collision gases other than helium which is, however, unique in its ability to induce process (iii), as well as in its appreciably larger cross section for process (ii). Thus the extrapolation to the origin (Figure 8) for process (ii), implying that this process is, like processes (i) and (iii), wholly collision-induced, is consistent with all the available experimental evidence. The only physically reasonable alternative would be that the background gas has essentially zero efficiency in inducing process (ii), (as for process (iii)), so the appropriate extrapolation to zero pressure, in Figure 8, would be parallel to the pressure axis. Such a procedure implies a truly unimolecular contribution to process
The Journal of Physical Chemistry, Vol. 90, No. 23, 1986 6097
The Dication of Nitric Oxide
TABLE I V Experimental Information on Ne+ technique used type of information translational energy loss transition energies between nondissociating states of NO2+ spectroscopy
double-charge-transfer spectroscopy Auger spectroscopy
appearance energy
translational energy release measurements on N O 2 + after a few ps translational energy release measurements on NOoZ+from time zero after formation emission intensity from NO2+
1000
F
'
*-W2t+Ot.T=SeV
1.7 f 0.1, 4.8 f 0.1 39.3 f 0.5 42.4 f 1.0 47.2 f 1.0 onset, 35.1 (35.7) transition energies between K-hole ions NOK+(and NK+O) and states of NO'", converted to transition energies from NO(X211) max, 37.0 (36.8) 40.2 (40.6) 43.7 (-) 48.0 (49.1) 54.6 (54.6) transition energy between NO'(X211) and NOa2+ions stable for a few ps 39.3, 41.2 39.8 f 0.3 38.3 f 0.5 39.4 f 0.1 38.6 f 0.1, 40.0 f 0.1 transition energy between dissociating levels of N O 2 + which survive a few ps, 6.1 f 0.1 and atomic fragments N + plus O'+ 8.1 & 0.1
33 15 34 41 42 35, 36 36
transition energy between dissociating levels of N O 2 + and atomic fragments N + plus O", measured in coincidence
9.0 f 0.5 5.0 f 0.3
37, 38 38
onset for emission in coincidence with ions of m / z 15
42.5 f 0.5
42
/ /
3 x
-
500
+
0
Figure 8. Variation of absolute intensities (arbitrary units) of collision-
induced dissociation processes of NO2+ with indicated pressure in the collision region. Accelerating potential 8 kV, electron energy 100 eV; data extracted from Figure 7. Triangles, dissociative electron capture (process (i)). Squares, direct dissociation, 6.1-eV energy release (process (ii)). Circles, direct dissociation, 8.1-eV energy release (process (5)). (ii). Unfortunately, the present experiments cannot give a wholly definitive answer to the question of the existence of a truly unimolecular process, due to our inability to control the pressure of background gas or to reduce it to much lower values. However, the fact that CO and similar collision gases do induce process (ii), with low efficiency, does favor the choice of no unimolecular contribution as the correct interpretation.
IV. Comparison with Experimental Data The available experimental data for NO2+are summarized in Table IV. Previous discussions have included an attempt36 to correlate the translational energy release measurements with the a b initio calculations due to Thulstrup et al.,I9 and a brief intercomparison of most of the data included in Table IV with particular reference to the translational energy loss experimenkW (37) Brehm, B.;de Frenes, G. Int. J . Mass Spectrom. Ion Phys. 1978.26, 251. _. .
(38) Curtis, D. M.; Eland, J. H. D. Int. J. Mass Spectrom. Ion Processes
1985, 63, 241.
ref 29 30
transition energies between NO'(X211) and doublet states of NO2+'
1500
;
transition energies, eV 1.74, 4.75
(39) Benoit, C.; Horsley, J. A. Mol. Phys. 1975, 30, 557. (40) Los,J.; Govers, T. R. In Collision Spectroscopy; Cooks, R. G.;Ed.; Plenum: New York, 1978.
31
32
The various categories of experiment are discussed in turn below, in the context of the a b initio calculations described above. a. Appearance Energies. For NOZ+ions to be detected in a typical mass spectrometer, they must have a lifetime of at least several microseconds and must therefore correspond either to the X22? or A211state. Since the u = 0 level of each of these states is accessible by Franck-Condon transitions from the u = 0 level of the XzII state of NO, it seems natural to assign the observed onset to formation of ground-state NO2+,viz., the u = 0 level of the X2Z+ state. The most recent electron impact ionization onset study33appeared to detect a break in the ionization efficiency curve, interpretable in terms of separated ionization energies of 39.3 and 41.2 eV. Alternatively the data could be i n t e r ~ r e t e d , ~ ~ within the experimental scatter, in terms of a single ionization onset a t 40.3 f 0.3 eV. The comparable experimental determ i n a t i o n ~ 'reported ~ , ~ ~ no similar break, and gave values of 39.8 f 0.315 and 38.3 f 0.5 This degree of agreement amongst different studies in significantly worse than normally expected for experiments of this kind. Since there appears to be no logical basis on which one determination may be preferred over the others, we are compelled to accept a best value of 39.5 f 1.0 eV, the average of the three independent determinations. Thus, for purposes of comparison, the u = 0 level of the X2Z+ state of NO2+ (calculated to have a value of -128.027 35 au absolute) was assigned an energy 39.5 eV above ground-state NO, and this was used to construct Tables I1 and I11 and Figures 1-4. The "absolute" energy values thus obtained must be used with caution, since a shift of the full theoretical manifold by f l eV relative to ground-state NO is conceivable. In this regard, two very recent photoionization s t ~ d i e s ~are ',~~ important. The photoionization threshold for formation of NO2+ was determined by Samson et al.41 to be 39.4 f 0.12 eV, in excellent agreement with the average value from the electron impact studies. However, detailed studg2 of the threshold region, using very high detection sensitivity, revealed a photoionization efficiency curve with a distinct break or change in slope, interpretable in terms of superposition of two independent transitions with onsets at 38.6 f 0.1 and at 4.0.0 f 0.1 eV. Above about 40 eV, the results obtained42 wpre in good agreement with those obtained41 at lower sensitivity. This result is qualitatively similar to that observed by Kim et al." using electron impact ionization, although the latter values are higher by about 1 eV. (41) Samson, J. A. R.; Masuoka, T.; Pareek, P. N. J. Chem. Phys. 1985, 83, 5531. (42) Besnard, M. J.; Hellner, L.; Mlinovich, Y.;Dujardin, G., to be pub-
lished.
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The Journal of Physical Chemistry, Vol. 90, No. 23, 19'86
This most recent photoionization must be accorded considerable weight, in view of the high sensitivity used and the unambiguous energy scale (compared with that in electron impact studies). In common with all studies of threshold energies, conducted at very high sensitivity, this faces problems of interpretation of the threshold region. For example, in the present case, low signal levels for ion of m / z 15 can be anticipated to arise from 15N+. Cross sections for photoproduction of N+ from N O have been determined by Samson et al.41and are greater than those for production of NOZ+,in the 39-40-eV range, by a factor of 100. Since the natural abundance ratio of l5N/I4N is 0.37 X as much as 40% of the signal observed at m / z 15 in this region could be due to 15N+rather than to I4NO2+. (States of NO+ in the region 37-48 eV have been observed in (e,2e) electron s p e c t r o ~ c o p y and , ~ ~ some of these are presumably the source of ~ ) possible perturbations liable to show the N+ ~ b s e r v e d . ~Other up at high sensitivity include hot bands due to photoionization of vibrationally excited NO. In fact, ion current for m / z 15 was detected4z for photon energies as low as 37.5 eV. Despite such uncertainties, the threshold region thus observed can be persuasively i n t e r ~ r e t e din~ ~terms of well-separated transitions to the XzZ+ and A 2 n quasi-bound states of NO2+. In the Franck-Condon region defined by ground-state NO, the present theoretical calculations predict a separation between these two states in the range 0.4 eV (outer turning point) to 1.8 eV (inner turning point). Thus, the separation of 1.4 eV observed42between the two ionization onsets is entirely consistent with such an interpretation. If valid, such a conclusion would imply that the present estimate (Tables I1 and 111, and Figures 1-4), that the u = 0 level of the X2Z+ state of N O lies 39.5 f 1.0 eV above ground-state NO, would have to be revised downward by 0.5-0.9 eV with some reduction in the uncertainty. A shift of 0.5 eV would not significantly affect the present interpretation of the Auger spectra,32which are unfortunately somewhat complicated, and would actually improve somewhat the interpretation of the kinetic energy release data which fix the energies of the dissociating states of NO2+relative to the dissociation limits. These phenomena are discussed further below. A feature of this most recent threshold curve is the excellent linearity observed42between 40 and 45 eV. N o further breaks were observed, suggesting that no other states of NOz+contribute significantly to photoionization processes producing stable NOZ+ (a few microseconds lifetime) in this range. This negative result is of significance for the character (quasi-bound or repulsive) of the first excited %+ state, as it suggests that dissociation is a dominant process for this state and that the efficiency of radiative stabilization, to produce detectable NOZ+,is at mast a few percent. This point will also be referred to below. b. Auger Spectroscopy. Only one report of the Auger spectrum of NO appears to be a ~ a i l a b l e .It~ ~is complicated by coupling of the 1s electron in the intermediate K-hole ion with the unpaired electron in the 2p x* molecular orbital. This spin coupling gives rise to singlets and triplets, with a corresponding splitting of 1.5 eV in the Auger lines excited by core ionization of the nitrogen atom, and about 0.7 eV in the case of oxygen. This complication has a potential positive feature, however, in that these splittings provide in principle a crucial test for lines to be assigned as Auger transition^.^^ On the basis of simple statistical weights of the singlet and triplet states of the K-hole ion intermediate, the intensity ratio is anti~ipated'~ to be 1:3. In addition, the comparison of Auger spectra excited by X-rays with those excited by energetic (keV) electrons provides a further confirmation of an observed line as a true Auger line. Unfortunately, due to the rather low energy resolution which could be achieved in these experiment^,^^ or possibly resulting from unknown nonstatistical behavior, the singlet-triplet splittings were not clearly apparent in all cases. In fact, it is clear from the published spectra32 that many peaks overlap with one another, making meaningful interpretation extremely difficult. For this reason, the partial interpretation presented here is not significantly altered if the energy scale, (43) Brion, C. E.;Tan, K.H.J . Electron Spectrosc. 1981, 23, 1.
Wetmore and Boyd adopted in Tables I1 and I11 and Figures 1-4, is shifted as discussed above. The band corresponding to highest energy of the Auger electrons, which appeared in spectra excited by both X-rays and kiloelectrovolt electrons, is that designated B-1 in the original work. When converted to energies of states of NOZ+, relative to ground-state NO via (assumed singlet state) K-hole energies, the values derived for the onset and peak maximum are those listed in Table IV. These values lie well below even the lowest estimate34 for the energy of ground-state NOz+ obtained by electron impact ionization. This observation raises doubts concerning the identification of line B-1 as a "normal" Auger line, corresponding only to removal of a K-shell electron and resulting in two vancies in weakly bound molecular orbitals in the product dication (a K-WW process32). A very weak shoulder (labeled B-2) could correspond to the triplet-state version of line B-1 in the nitrogen Auger spectrum, but no analogous line was apparent in the oxygen Auger spectrum3z even though there are no interfering peaks in the spectrum in the predicted region. Accordingly, the most probable interpretation of line B-1 (Table IV) is in terms of monopole excitation (electron shake-up) accompanying the K-shell ionization, Le., a KWe-WW process, as suggested in the original Such a process would show up in Auger spectra excited both by X-rays and by kiloelectronvolt electrons. On the other hand, if this process were to occur by relaxation of an excited intermediate, and the final state were the lowest state of the doubly charged ion, then the difference between the B-1 and B-3 energies (-5 eV) would be a measure of this internal excitation. Applying this same reasoning to B-2 would imply a final state 1.1-1.9 eV above the ground state. The 211 state lies at 0.5-1.8 eV in the FC region. While loss of a x electron might prove to be a less efficient process, this could provide an alternative that has interesting consequences for other states. However, the B-1 line is rather weak,3z in any event. The first intense lines are those labeled B-3 and B-5 (in the tabulated results, B-4 in the spectrum3z) for the nitrogen Auger spectrum and B-4 and B-5 in the oxygen spectrum. While these peaks also fail to show the 3:l intensity ratio anticipated for tripletsinglet pairs (thus suggesting overlap with other processes), it does seem possible to interpret them in a self-consistent fashion in terms of a normal K-WW Auger process. Thus, the following results are obtained for the combinations of singlet-triplet pairs with nitrogen-oxygen K-shell ionization. Nitrogen: singlet (B-3), 40.1 eV; triplet (B-4 or B-5), 40.3 eV. Oxygen: singlet (B-4), 40.7 eV; triplet (B-5), 40.4 eV. The agreement between these values (peak-maxima only), obtained for the energy of an (assumed) common final state of NOz+, is well within the experimental uncertainty. The process which interferes to alter the intensity ratios (singlet component too intense) must lie at an energy indistinguishable from that of the singlet component. Since the peak onsets correspond to states of NOz+ some 0.5 eV lower than those deduced from the peak maxima considered above, in good agreement with the electron impact onsets (Table IV), it is reasonable to assign these Auger bands to formation of NOZ+in the ground-state ( u = 0 level of %+). The nitrogen Auger spectrum of NO contains two bands (labeled B-6 and B-7 in the original work32) which possess all anticipated attributes of Auger transitions from a singlet-triplet pair of K-hole intermediate ions, leading to the same final state of NOz+. The values thus predicted for the energy of this state of NOz+, relative to ground-state NO, are 43.9 and 43.4 eV, Le., some 3.5 eV above ground-state NO2+. Unfortunately, no corresponding feature is apparent in the oxygen Auger spectrum of NO,32although a broad shoulder labeled B-8 in this spectrum might correspond. These two features do map onto B-9 by the same reasoning applied to B-1 and B-3, viz., a mechanism involving an excited K-hole intermediate. An intense band, labeled B-9 in both oxygen and nitrogen Auger spectra,32is clearly a composite peak. If the observed electron energies are converted via an assumption of singlet K-hole intermediates, the energies of the final state of NOZ+thus obtained
The Dication of Nitric Oxide
The Journal of Physical Chemistry, Vol. 90, No. 23, 1986 6099
are 49.1 and 48.0 eV, (measured at peak maxima). The agreement is probably as good as can be expected and leads to the conclusion of a group of states lying some 9 eV above ground-state NO2+. Note that since the entity observed directly in Auger ~pectroscopy~~ is the fast Auger electron, the states of NO2+ thus accessed need not be bound in any sense. The only other feature of these Auger spectra, which might be interpreted in terms of normal K-WW transitions, is the poorly resolved pair of peaks labeled B-13 and B-14 in both the nitrogen and the oxygen Auger spectra. On the basis of assuming these features to correspond to a singlet-triplet pair, as discussed above, the following energies for the (assumed) common final state of NOz+may be obtained. Nitrogen: singlet (B-13), 54.7 eV; triplet (B-14), 54.5 eV. Oxygen: singlet (B-13), 54.2 eV; triplet (B-14), 55.0 eV. Again, the agreement is satisfactory within the experimental uncertainties and indicates Auger transitions to a group of states some 15 eV above ground-state NO2+. The interpretation of all of this information,3zin terms of the present theoretical calculations, is not wholly satisfactory. As discussed above, the B-1 and B-2 bands probably do not correspond to normal Auger processes, so the corresponding very low energy states of NO2+,inferred in Table IV, are spurious. The next higher state listed in Table IV (some 40 eV above ground-state NO) can be satisfactorily interpreted in terms of formation of ground-state N@+, though the experimental energy resolution32was sufficiently low that any contributions from the AzII state could not have been resolved. The singlet-triplet pair of lines B-6 and B-7, observedj2 in the nitrogen Auger spectrum only, correspond to a state of NO2+lying 43.7 f 0.2 eV above ground-state NO. The best theoretical interpretation of this observation would be in terms of the lowest 42+state (Figures 2-4), for which the computed energies at the two Franck-Condon limits defined by ground-state NO are 45.4 and 44.5 eV. The fact that these bands were not observed3z in the corresponding oxygen Auger spectrum raises suspicions that they may not corresporid to a normal K-WW process as was implicitly assumed in order to convert the observed electron ene r g i e to ~ ~energies ~ of states of NOz+. This rather poor agreement between theory and experiment is improved if lines B-1, B-2, B-6, and B-7 are shifted by a common factor of 5 eV to account for such a process, as discussed above. The group of states of NOz+, which correspond to the experimenta132 lines B-9, must have energies in the range 48-49 eV in the Franck-Condon region. A large number of potential curves cross the Franck-Condon region in this energy range, thus accounting for both the large intensity of the B-9 lines as well as their broad, composite nature.3z It is much more difficult, however, to account for lines B-13 and B-14, which correspond to states of NOz+ at 54.6 eV within the Franck-Condon range. While there are many states listed in Table I1 (only a few of which are shown in Figures 1-4) which possess appropriate energies in the Franck-Condon range, there exist similar numbers of states both above and below this energy. It is thus difficult to account for the fairly well-defined features of B-13 and B-14 observed.32 In summary, while the Auger spectra of NO32can be satisfyingly interpreted to confirm the energy of ground-state NOz+to lie within the (unfortunately rather wide) limits established by ionization onsets (Table IV), other features of these spectra are not well understood. Quite apart from theoretical problems, it appears that more phenomenological problems of interpretation exist which make the N O case considerably more complicated than are those of C O and N2, for example. The Kronig selection rules,' appropriate to the radiationless Auger transitions, are of little practical assistance in the interpretation since the angular momenta of the emitted Auger electron must be taken into account. c. Double-Charge-Transfer Spectroscopy. As in the case of Auger spectroscopy, discussed above, this technique can access both bound and repulsive states of the product dication since only the fast hydride ion is observed directly.31 However, for the first-row elements of interest here the total spin is conserved to
a good approximation, so that only doublet states of NO2+ can be accessed in the essentially Franck-Condon transition^.^^ The lowest state of NO2+ thus observed (Table IV) is at an energy in acceptable agreement with that inferred from both electron impact ionization onsets and from Auger spectroscopy. This is reassuring, in view of questions raised39concerning the absolute calibration of the energy scale in these experiments. The first excitation of NO2+,reported in the double-chargetransfer spectroscopy experiments, corresponds to a very weak, barely perceptible bump in the translational energy spectrum of the fast product hydride ions.31 It is not possible to interpret this feature in terms of any excited state (doublet or quartet) of NOZ+ calculated in the present work. In fact, examination of the published spectrum indicates that two weak bumps, of roughly equal intensities, are in fact present whereas only one was tabulated.31 In the absence of any further information, it seems permissible to assign these bumps to experimental artifacts of some kind (such as Nzand O2impurities in the NO sample), or even to noise. The third reported peak, however, is well-defined and clearly significant. The quoted uncertainty (Table IV) is that associated with establishing the position of the peak maximum, but the peak itself is about 4-eV wide (full width at half-maximum). It seems likely that this peak can be assigned to transitions to the group of states (of which several are doublets) which cross the Franck-Condon region in the energy range 47-49 eV, and which were invoked above to account for peak B-9 in the Auger spectra. In this respect, at least, the present theoretical results offer a satisfying reconciliation of different experimental data. d . Translational Energy Loss Spectroscopy. This title is usually reserved for experiments in which a kiloelectronvolt beam of a species such as NOz+ is made to collide with a gas molecule, and events which result in no chemical change (Le., internal state changes only) are recorded. In a wider sense double-chargetransfer spectroscopy could be included in this category, but since it involves change of the H+ projectile to H-, it is usually excluded. Two such independent i n v e s t i g a t i ~ n of s ~collisional ~ ~ ~ ~ excitation of NOz+ (without dissociation) gave identical results (Table IV). In order to account for the experimental o b s e r v a t i ~ n its ~is ~ ~ ~ ~ necessary to devise processes which start with states of NOz+which can survive for a few microseconds, then undergo collisional excitations of the appropriate energies (Table IV), and subsequently remain bound for the subsequent few microseconds required for the excited NOz+ ions to exit from the translational energy analyser. The transition requiring 1.7 eV (Table IV) is readily accounted for by a vertical excitation from the v = 0 level of the XzZ+ state to the u = 7 level of the AZII state. The predicted excitation energy is 1.5 eV, and the tunnelling lifetime of the product level is about s, though a more likely fate of this product is stabilization via wholly allowed radiation back to the ground state. The excitation requiring 4.8 eV is a little more difficult to explain. Collisional excitation from the inner turning points of the u = 1 or u = 2 levels of the X2X+state, both of which lie below the crossing with the AZII state and thus are not subject to radiative deexcitation, can reach the repulsive wall of the AZIIstate with energy changes in the range 3.7-4.0 eV. The discrepancy between this theoretical prediction and the experimental value is a little large in view of the anticipated accuracy for the present ab initio calculations. In order that such collisionally excited NOZ+ ions may survive the few microseconds to the detector, radiative deexcitation to the ground state wouid have to compete effectively with dissociation. An alternative interpretation is possible if one considers collisional excitation of the u = 0 level of the AzII state, which should also have a significant population by the time molecules reach the collision region. A transition from the outer turning point of the u = 0 level of the A211 state to the first excited zII is predicted (Tables I1 and 111) to require 4.8 eV. The latter has a broad almost flat region (Figures 1 and 4), so that nuclear separation would be sufficiently slow that allowed radiative decay could compete effectively. The absence of any bound states in
6100
The Journal of Physical Chemistry, Vol. 90, No. 23, 1986
this energy region does seem to require that the excited state initially formed rapidly decay to one of the two known stable states in order to survive until detection. In this context, it seems appropriate to comment upon earlier theoretical of NOZ+.The semiempirical procedure of Hurley18 predicts quasi-bound states X2Z+ and A211, in qualitative agreement with the present work and with the preV ~ O U S 'CI ~ calculations which used a limited basis set. However, HurleyI8 also predicted a quasi-bound B2Z+state, with an R, value very close to that of the ground state and about 4 eV higher in energy. This finding is highly suggestive in view of the nondissociative excitation of about 4.8 eV o b ~ e r v e dby ~ ~translation,~~ al-energy-loss spectroscopy. However, the earlier ab initio study due to Thulstrup et al.I9 found only a very shallow well for this state, at a much larger value of Re such that a vertical excitation would reach only the repulsive wall of the potential curve. The much more reliable ab initio calculations reported here finds this first excited 2E+state to be wholly repulsive (Figure 1 and Table 11). The trend in such calculations is from a simple bonding plus 1 / R model (Hurley) to one with some correlation flexibility to one in which correlation and polarization effects are strongly addressed. Lowering the CP limit states by accurate description of long-range electrostatic interactions would reduce barriers and shift minima to larger R values. Accordingly, it is believed that the experimentally o b ~ e r v e dexcitation ~ ~ . ~ ~ of 4.8 eV must correspond to radiative relaxation of the excited state initially formed, as proposed above, and that the semiempirical procedure of Hurley18 is in error in this case. This question of the nature of the excited *Z+ state is discussed further below. e. Translational Energy Release, Microsecond Delay. The experiments to be described under this subheading involve formation of NO2+ions by electron impact ionization, their subsequent acceleration to a beam of ions in the kiloelectronvolt energy range, followed by study of their dissociation reactions (spontaneous or collision induced) over a time window of a few microseconds which typically starts 2 or 3 ps after formation. The important experimental parameter which is measured in these experiments is the translational energy released to the fragments. In cases such as the present one, where the reactant ion is diatomic, the measurement amounts to locating the energy of the dissociating level@) above the appropriate dissociation limit. The first work of this kind was due to Beynon et who reported a unimolecular dissociation occurring in the time window 2-4 ps after the formation of the NO2+ precursor ions, with a translational energy release of 6.15 eV. This work was later repeated36with similar apparatus, and a value of 6.1 f 0.1 eV was deduced from an extremely weak signal. Indeed, some considerable doubt was expressed36as to whether the observations correspond to a unimolecular dissociation at all. The present experiments, reported above, were undertaken to resolve this ambiguity. The most reasonable interpretation of the present experimental work (Figure 8) is that no unimolecular component exists, though it cannot be entirely ruled out. This wholly empirical conclusion is in accord with predictions based upon the present theoretical calculations. Thus the only states of NO2+,predicted to survive for the few microseconds required to reach the experimentally observable region, are the v = 0, 1, and 2 levels of the X2Z+state and the u = 0 level of the A211state, as discussed above. The tunnelling lifetimes of these levels (Table 111) are effectively infinite for these experiments. Rotational levels within the u = 14 and 15 manifolds of the X2Z+ state, and within the u = 6 and 7 manifolds of the A211state, do have tunnelling lifetimes in the microsecond range appropriate for unimolecular predissociation to be observable under the experimental conditions of interest. However, the radiative lifetimes of these levels are anticipated to be much shorter (- IO9 s), so that no tunnelling predissociation can be predicted to be observable on a microsecond time scale. The only possibility for a unimolecular electronic predissociation mechanism involves crossing of the X2Z+ state by a repulsive 411 state at 1.5 A (Figure 4). Such a spin-forbidden process would be anticipated to proceed on a microsecond time scale, but again the radiative lifetimes of the
Wetmore and Boyd appropriate levels of the XzZ+ state should be far shorter. Collision-induced dissociation of those NO2+ions, which have survived some 2 ps after formation, must involve collisional excitation of the low vibrational levels of the X22+and A 2 n states. The experimental time window for dissociation subsequent to collision lies in the range from effectively zero to a few microseconds. Mechanisms for collision-induced d i s ~ o c i a t i o nmay ~ ~ be roughly divided into direct electronic excitation to a repulsive state, electronic (plus possibly rotational-vibrational) excitation to a quasi-bound state of appropriate predissociation lifetime, and purely rotational-vibrational excitation to levels which are predissociated. The predissociation mechanism may be either curve-crossing (case I)' or by tunnelling (case HI).] The collision-induced process at 8.1 eV (Table IV) most probably involves collisional excitation to levels of the X2X+ state whose tunnelling lifetimes (Table 111) are short enough to compete with radiative decay. Such levels are probably in the u = 16 and 17 manifolds (Table 111) which are calculated to lie 8.3-8.4 eV above the dissociation limits. The prediction is in acceptable agreement with experiment. Similarly, the process with a translational energy release of 6.1 eV (Table IV) probably corresponds to tunnelling from the u = 8 and 9 levels of the A211state. The predicted translational energy release to ground state N+ plus ' 0 is 6.3-6.4 eV. The predicted values are slightly high, and the agreement would be improved by a downward shift of the absolute energy scale, as discussed in (a) above. In both cases the competition between radiative decay and dissociation serves to narrow the range of levels which can contribute to the observable dissociation processes. The shapes of the peaks in the translational energy spectra (Figures 5 and 6) indicate that fairly narrow ranges of values of translational energy release are involved. The underlying continuum, with values of translational energy release up to 15 eV or so, probably involves excitation of the lowest levels of the two quasi-bound states to the repulsive curves which cross the appropriate region between about 44 and 52 eV (Figure 3). Since each of the three lowest asymptotic dissociation levels (Table I) correlates with at least one of these repulsive curves, a broad continuum of translational energy release values, ranging from about 7 to 16 eV, is easily predicted for such direct mechanisms. This is in reasonable agreement with observation. It remains to discuss briefly some more qualitative observations concerning the collision-induced processes. The unique properties of helium as a collision gas for these and related processes have been discussed previously36and are probably connected in some way with the large binding energies of all electrons in helium. In any event it is clear that, for collision energies in the kiloelectronvolt range, helium is much more efficient than any other collision gas in inducing processes requiring larger energy transformations. This is in accord with the observations summarized in Figure 8. Thus, the much larger excitation energy ( 3.4 eV from u = 0 to u = 17 of the X2Z+ state) required to excite process (iii) (8.1 eV energy release) compared with that (- 1.2 eV from u = 0 to u = 9 of the A211state) for process (ii) (6.1 eV energy release) is entirely consistent with the observation (Figure 8) that the former is apparently induced by collisions with helium only while the latter is also induced (with lower efficiency) by collisions with background gas. f. Measurements of Translational Energy Release in a Time Window Extending from Zero Time after Ionization. The experiments of interest in this category generally involve ion-ion coincidence detection. The first such experiment on NO2+ N+ +0 ' was due to Brehm and de F r e n e ~ who , ~ ~ measured the translational energy spectrum using a retardation technique requiring differentiation of the raw data. The resulting distribution of translational energy release was broad and featureless, extending from about 5 to 23 eV with a badly defined maximum near 8-9 eV. A more recent coincidence experiment on NO2+ used time-of-flight differences to characterize the dissociation process38. Several dications were studied by using this technique.38 While no translational energy spectrum was published for the NO2+ case,38tabulated results gave maxima centered at 5 and 9 eV with full width at half-maximum values of 2.3 and 4 eV, respectively.
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The Dication of Nitric Oxide The latter of these two maxima38presumably corresponds to that observed in the earlier in which the energy resolution was not adequate to resolve the maximum at lower energies. Since the observation time window in these experiment^^^,^^ started at much shorter times after formation of the NO2+ions, events proceeding from repulsive states can be invoked. Indeed, the lowest value ~ b s e r v e d ~(
