J. Phys. Chem. 1995,99, 15775-15778
15775
Threshold Photoelectron Spectrum of Molecular Oxygen in the Region of the B 2Zi X 3Eg Transition?
-
F. Merkt' and P. M. Guyon* Laboratoire des Collisions Atomiques et Molbculaires, Unit6 de Recherche Associ6e au CNRS 281, Universiti de Paris-Sud, Bat. 351, F-91405 Orsay, France Received: April 26, 1995; In Final Form: August I , 1 9 9 9 The threshold photoelectron (TPE)spectrum of isotopically substituted molecular oxygen (1802) has been measured in the region 20.2-21.6 eV in order to determine the origin of the new progression recently observed by Baltzer et al. (Phys. Rev. A 1992, 45, 4374) by He I1 photoelectron spectroscopy and by Merkt et al. (Chem. Phys. 1993, 137, 479) by threshold photoelectron spectroscopy. The adiabatic ionization potential for this progression is determined to be 20.352 f 0.002 eV for 1 6 0 2 . Intensity perturbations in the spectrum of the B X '2; are also reported.
-
2ii
-
1. Introduction Recent measurements of the He I1 photoelectron spectrum,' and of the threshold photoelectron spectrum2s3of molecular oxygen have revealed a new progression between 20 and 22 eV. Owing to its weakness and to its partial overlap with the B X '2; transition, this progression had not been observed in previous lower resolution photoelectron spectra. Although a comparison with an ab-initio calculation suggested that this progression might be assigned to the 2 211u X 'E; transition, the adiabatic ionization potential calculated abinitio was found at lower energy than the first member of the progression observed experimentally.' The purpose of this report is to resolve this discrepancy and determine the adiabatic ionization potential for this progression from a measurement of the isotopic shifts in the threshold photoelectron spectrum of the 1 8 0 2 sample. The recording of the TPE spectra of the 1 8 0 2 isotope in this region also provides a good opportunity to extend our investigations of intensity distributions in the threshold ionization of molecular ~ x y g e n . ~ Strong , ~ intensity perturbations are reported which lead to a pronounced non-Franck-Condon behavior in the threshold photoelectron spectra of the B-X transition of the 1 6 0 2 and 1 8 0 2 isotopes. 2. Experimental Section The data have been recorded at Super-ACO, the Orsay synchrotron radiation positron storage ring, using a 3 m normal incidence monochromator to disperse the VUV radiation. The monochromator, equipped with a 1200 linedmm holographic grating and 50 pm slits provided a photon bandwidth of 0.02 nm. The double time of flight spectrometer used in the present experiment, which provided a maximum resolution of 5 meV, is the same as that previously used to study the TPE-ZEKE photoelectron spectrum of nitrogens and will not be described here. However, instead of using a pulsed electric field to extract the threshold electrons as in ref 5, we use a dc field of 0.4 V/cm. The selection of threshold electrons is achieved by angular and time of flight discrimination as in ref 2.
1, respectively. Two progressions can be recognized in Figure l:, the dominant one which corresponds to the B 22,X 32,- transition and a weaker one which is the object of the present investigation. The positions and intensities of the members of the B-X progression derived from the spectra in Figure 1 are compared in Table 1 with the results obtained by Baltzer et al.' by He II photoelectron spectroscopy. Table 2 summarizes the line positions measured for the second progression observed in Figure 1. There is good agreement between the line positions measured for both progressions in the He I1 and the threshold photoelectron spectra of the 1 6 0 2 isotope. The intensity distributions in the B-X progression, however, differ significantly. The most intense line corresponds to the transition to the v+ = 0 level in the threshold photoelectron spectra, but to the v+ = 1 in the He I1 photoelectron spectrum. The highest vibrational levels (v+ = 5, 6) appear abnormally strong in the TPES. The v+ = 4 level, on the other hand, has a particularly weak intensity in the threshold photoelectron spectra, particularly so for the 1 8 0 2 isotope. As expected, the positions derived from the threshold photoelectron spectrum of the 1 8 0 2 isotope are shifted with respect to those measured on the 1 6 0 2 isotope. Figure 1 and Tables 1 and 2 form the basis of the analysis and discussion presented below. (A) Determination of the Origin of the New Progression. The shift in the positions of the transitions measured in the spectrum of 1 8 0 2 with respect to those measured in the spectrum of the 1 6 0 2 isotope can be estimated from eqs 1-4 below, which express the vibrational constant we, the anharmonic constant ode,the zero-point vibrational energy El12, and the ionization potential IP of the 1 8 0 2 (or isotope in terms of the constants of the ' 6 0 2 (or ' 6 0 ; ) isotope.
3. Results and Discussion The threshold photoelectron spectra of 1 6 0 2 and 1 8 0 2 in the region 20.2-21.6 eV are displayed in parts a and b of Figure
(3)
22i
-
-
' Dedicated to Professor Zdenek Herman on the occasion of his 60th
birthday. This paper was originally submitted for the Zdenek Herman Festschrift [J. Phys. Chem. 1995,99 (42)l. - Present address: Physical and Theoretical Chemistry Laboratory, Oxford OX1 342, U.K. Abstract published in A h a n c e ACS Abstracts, October 1, 1995. @
w.pe(ls02)= 8/90.p~('~O~)
IP('~O,, new state)
- IP('~o,, new state) = AE,,,(x 'E;) - AE,,,(new state) (4)
The following procedure was followed to determine the origin of the progression. First, the line positions of both isotopes were least-squares fitted to a second-order polynomial in the
0022-3654/95/2099-15775$09.00/0 0 1995 American Chemical Society
15776 J. Phys. Chem., Vol. 99, No. 43, 1995
Merkt and Guyon TABLE 2: Line Positions of the Members of the New Progression Derived from the TPE Spectra of 1 6 0 2 and l S 0 2 and from the He II PE Spectrum' of 1 6 0 2 '602
U+
20.2
20.4
20.6
20.8
21.0
21.2
21.4
21.6
2i.2
21.4
2i.6
Energy (eV)
(b)
v)
c
c
B2
1 1 0
1
2
y -
3
x3ci
4
a
8 5 0
L
c
0
a
1
20.2
20.4
20.6
20.8
21.0
Energy (eV) Figure 1. (a) Threshold photoelectron spectrum of I6O2in the region 20.2-21.6 eV. (b) Threshold photoelectron spectrum of ' * 0 2 in the region 20.2-21.6 eV.
-
TABLE 1: Comparison of the Positions and Intensities of the Members of the B zZg- X 3Z- Progression Derived from the TPE Spectra of l 6 0 2 and h 0 2 and from the He I1 PE Spectrum' of l 6 0 2 '602
this work U+
0 1 2 3 4 5 6 7
Baltzer et al.'
this work
transition relative transition relative transition relative energy (eV) intensity energy (eV) intensity energy (eV) intensity 20.296 20.436 20.565 20.691 20.812 20.927 21.038 21.143
108 100 85 44 11.1 26.6 10.8 =l
20.296 20.433 20.563 20.690 20.812 20.928 21.040 21.146
84 100 70 38 18 7.9 3.4 -1
20.298 20.428 20.554 20.673 20.788 20.901 21.005 21.114
+
114 100 93 41 3.2 24 14.2 =l
vibrational quantum number u+ (Y = IP (we - w&V+ @&af2) for several different assignments, leading to a set of effective constants (IP, we, and wpre)for the ionic state. Then, the internal consistency of the sets of constants determined for both isotopes was tested with the use of eqs 1-4 above. The only vibrational assignment compatible with the line positions given in Table 2 corresponds to that given in the first column of this table. The following constants are obtained for the new ionic state of the 1602isotope: IP = 20.352 & 0.002 eV, we = 810 f 25 cm-', and w d e = 12 f 4 cm-I. The relatively large uncertainties in the determination of the vibrational and anharmonic constants originate from the restricted number of lines
this work transition energy (eV)
0 1
20.352 20.449
2 3 4
20.637 -20.725
Baltzer et al.' transition energy (eV)
'*02this work transition energy (eV)
20.351 20.450 ~20.544 20.637 20.726
20.354 20.446 -20.530 20.623 20.702
used in the fit. The unambiguous determination of the adiabatic ionization potential for this progression and the conclusion that the first vibrational members of this progression are observed both in TPE and He I1 PE spectra provide important clues for its assignment. Several ab-initio calculations of the excited bound electronic states of the 0 2 + ion have been reported,'S6-* starting with the work of Dixon and Hulls6 Consideration of the potential curves presented in these references for the accessible ionic electronic states in this energy range leads to the following conclusions concerning a possible assignment of the progression: (1) The adiabatic ionization potential for the 2 211,-X 3,2gtransition is calculated around 19.5 eV, i.e. lower than the experimental value by more than 0.5 eV. Moreover, the Franck-Condon factors for excitation to the lowest vibrational states of the 2 211ustate are expected to be very low indeed. Both observations render this assignment, suggested in ref 1, unlikely. (2) Although the calculated adiabatic ionization potential for the 2 211,-X 3Xgg-transition would be in close agreement with the values determined experimentally, the Franck-Condon factors ought to be vanishing for the lowest vibrational levels, and this assignment, proposed by us on energy gounds only,2 also appears unlikely. (3) Other electronic states of the 0 2 + ion are located in this energy range, but in general the potential curves have their minimum at larger internuclear distances than the ground neutral state, rendering the Franck-Condon factors extremely small near the origin of the progression. The first interacting two states of 2Xg- symmetry fonn perhaps an exception, but the adiabatic curve which would have appreciable Franck-Condon factors near the band origin does not appear to be able to sustain the four bound vibrational levels observed experimentally with certainty. The assignment of this progression therefore has to remain uncertain until further experimental and theoretical evidence is presented. (B)Intensity Distribution in the B X 3;Progression. The relative intensities of the members of the B-X progression measured by He I1 PES and TPES given in Table 1 are represented graphically in Figure 2. The intensities measured by He I1 PES differ from those measured by TPES in several respects. First, the transition to the v+ = 1 level is the most intense in the He Il PE spectrum, whereas the intensities in the TPE spectrum of both isotopes decrease monotonously between v+ = 0 and v+ = 3. Secondly, the intensities of the transitions to the vc = 4 level appear weaker in the threshold photoelectron spectra. Particularly striking is the almost vanishing intensity of the v+ = 4 band in the TPE spectrum of the 1802 isotope. Finally, the intensities of the u+ = 5 and v+ = 6 are considerably more intense in the threshold photoelectron spectra. Because intensities measured by He I1 PE spectroscopy usually reflect the direct partial ionization cross section ac-
-
J. Phys. Chem., Vol. 99, No. 43, I995 15777
Threshold Photoelectron Spectrum of Molecular Oxygen
"
0
1
2
3
v
4
5
6
-
Figure 2. Relative intensities of the members of the B 2&X 3&transition derived from the TPE spectrum of '602 and 1802 and from the He I1 PE spectrum of 1602 (from ref 1).
curately, it must be concluded that the intensities determined in the threshold photoelectron spectra are perturbed. Moreover, the non-Franck-Condon behavior observed in the TPE spectra cannot be attributed to a perturbation of the B state by a neighboring ionic state because, in this case, the intensities measured by He I1 photoelectron spectroscopy would be perturbed in the same fashion. Differences between intensities measured by TPES and He I1 PES could originate from a shape resonance, from a Cooper minimum, or from the interaction between a neutral state and the ionization continua located just above the successive vibrational levels of the B state of the ion. Raseev et aL9have investigated the effect of shape resonances in the 3a, photoionization of 0 2 and reported branching ratios for the formation of the v+ = 0, 1,2, and 3 levels of the B state at different excitation energies. At the lowest energies, i.e., nearest to threshold, their results show a regular decrease of the cross section with increasing vibrational quantum number of the B state and appear to reproduce the intensities in the TPE spectrum at least qualitatively. Unfortunately, no data are presented for the formation of the higher vibrational members of the B state. The sudden decrease in intensity observed in the TPE spectra near the v+ = 4 level of the B state and the strong isotopic dependence observed for this level indicate a very localized intensity perturbation, incompatible with the expected smooth variation of the shape resonance with internuclear distance. Alternatively, the relative weakness of the v+ = 4 members of the B-X progression could be due to a depletion of intensity caused by a window resonance. The photoabsorptionlo and photoionization' I cross sections both show a window resonance in the region of the v+ = 3 and 4 levels, centered at -594.5 A and extending up to 596 A. This window resonance has been assigned to a transition to the n = 3 sa, 3Z,,- Rydberg state converging to the c 4Z-v+ = 0 state of 0 2 + . The reduced photoabsorption cross section resulting from this window resonance seems at first sight an attractive explanation of the reduced intensity of the v+ = 4 members of the B-X progression in the TPE spectra. But its position lies at slightly shorter wavelength than the v+ = 4 band (595.73 A for 1602) in the TPE spectrum. In addition, assigning the weak intensity of the v+ = 4 band to this window resonance fails to account for the observed isotopic effect. Indeed, from the similar vibrational constants of the X state of 1602 and the c state of 0 2 + one does not expect a significant shift of the position of the n = 3 sa, 3Z,,- Rydberg state in the spectrum of 1802. The position of the v+ = 4 state in the TPE spectrum, on the other hand, is shifted to 596.43 A and lies further away from the
window resonance than in the case of the 1602 isotope. If the reduction of intensity in TPE spectrahad its origin in the reduced photoabsorption cross section caused by the n = 3 sa, 3Z,,window resonance, the v+ = 4 band ought to have appeared stronger in the spectrum of 1802 than in that of 1602, in contrast to the experimental results. It seems therefore unlikely that the window resonance is responsible for the intensity depletion observed in the TPE spectrum. The intensity enhancement of the v+ = 5 and 6 members of the B state, as well as the monotonously decreasing intensity observed between v+ = 0 and 2 in the TPE spectrum may originate from channel interactions between the ionization continua located immediately above the vibrational levels of the ion and low Rydberg states belonging to series converging to higher lying vibrational levels of the B state. Other resonances associated with n = 3 and 4 Rydberg states converging to the 3 *nu state of the ion are lying in the vicinity of the first vibrational levels of the B state. Although these states cannot be excited directly because of poor FranckCondon factors, they could perturb the Rydberg series converging to the v+ = 4-7 levels of the B state and thereby contribute to distort the intensities measured by threshold photoelectron spectroscopy. One may also consider that the Rydberg series converging to the v+ = 5-7 levels of the B state, which can contribute to the intensity of the v+ = 4 member in the TPE spectrum by a channel interaction, could be predissociated by the repulsive part of the potential of the n = 3 sa, 3Z,,- Rydberg state, which is responsible for the window resonance discussed above. However, if one shifts the potential of the c 42,,- state of 0 2 + (to which the Rydberg series converges) calculated by Tanaka et all2 by the difference between the c 42,,- v+ = 0 state of 02+and the n = 3 sa, 3X,,- (v = 0) resonance center, the repulsive part of the potential energy curve of the resonance intersects the B state outer potential wall in the vicinity of the v+ = 2 vibrational level and not at v+ = 4. Nevertheless, the exact potential may be slightly different from that deduced form this simple translation and the possibility of the curve crossing occurring between the v+ = 3 and v+ = 4, which would explain both the intensity depletion of the v+ = 4 level in the TPE spectrum and the pronounced isotope effect, cannot be ruled out. 4. Conclusions
The threshold photoelectron spectrum of 1802 has been recorded between 20.2 and 21.6 eV. The origin of the new progression recently observed by He III and threshold photoelectron spectros~opy~*~ is determined to be at 20.352 f 0.002 eV. The vibrational constant determined for the ionic states amounts to me = 810 f 25 cm-I. The intensity distribution in the TPE spectrum of the B 2&X 3&- transition reveals a certain number of differences compared to those measured in the He I1 photoelectron spectrum. Possible interpretations of these differences are discussed.
-
Acknowledgment. This work was partially funded by the EU Research Program Human Capital and Mobility Program, Network Structure and Reactivity of Molecular Ions, No. ERB CHRX CT 930 150. We are grateful to the staff at LURE for technical assistance and for operating the SUPER-ACO storage ring. We thank Dr. C. Alcaraz, Dr. 0. Dutuit, and Dr. H. Lefebvre-Brion for experimental help and discussions. F.M. gratefully acknowledges financial support from the Minist8re des Affaires Etranghes, France, and St. John's College, Oxford.
15778 J. Phys. Chem., Vol. 99,No. 43, 1995
References and Notes (1) Baltzer, P.; Wannberg, B.; Karlsson, L.; Carlsson Gbthe, M.; Larsson, M. Phys. Rev. A 1992, 45, 4374. (2) Merkt, F.; Guyon, P. M.; Hepburn, J. W. Chem. Phys. 1993, 173, 479.
(3) Ellis. K.: Hall, R. I.: Alvadi, L.; Dawber, G.; McKonkev, A.; Andric, L.; King,G. C . J. Phys. B 1994, 27, 3415. (4) Guvon, P. M.; Hepbum, J. W.; Weng, - T.; Heizer, F.;Reynolds, D. Phys. Rev. h t . 1991, 67,-675. (5) Merkt, F.; Guyon, P. M. J. Chem. Phys. 1993, 99, 3400. (6) Dixon, R. N.; Hull, S. E. Chem. Phys. Lett. 1969, 3, 367.
Merkt and Guyon (7) Beebe, N. H. F.; Thulstrup, E. W.; Andersen, A. J. Chem. Phys. 1976, 64, 2080.
(8) Marian, C . M.; Marian, R.; Peyerimhoff, S. D.; Hess, B. A.; Buenker, R. J.; Seger, G. Mol. Phys. 1982, 46, 779. (9) Raseev, G.; Lefebvre-Brion, H.; Le Rouzo, H.; Roche, A. L. J. Chem. Phys. 1981, 74, 6686. (10) Holland, D. M. P.; Shaw, D. A,; MacSweeney, S.M.; MacDonald, M. A,; Hopkirk, A.; Hayes, M. A. Chem. Phys. 1993, 173, 315. (11) Dehmer, P. M.; Chupka, W. A. J. Chem. Phys. 1975, 62,4525. (12) Tanaka,K.; Yoshimine, M. J. Chem. Phys. 1979, 70, 1626.
JP951172Z
