J. Phys. Chem. 1985, 89, 4213-4219 There is every reason to continue both theoretical and experimental work in an effort to converge on an increasingly accurate value of the absolute potential of the standard hydrogen electrode.
Acknowledgment. Supported in part by N S F Grant CHE8207432. We are most grateful to Professor A. 0,Petrii of Lom o n m v Moscow State University for his advice on issues relating to the potential of zero charge. We also thank R. C. Frye for
-
4213
barrier height measurements, D. B. Colavito for e-beam evaporation of platinum films, J. D. Porter and D. E. Aspnes for advice on the surface chemistry of InP and on the preparation of the crystal faces, and S . H. Glarum and Professor Roger Parsons for reading and criticizing the manuscript. Registry No. H2, 1333-74-0; InP, 22398-80-7; Pt, 7440-06-4.
Emission Spectra of Supersonically Cooled Halocyanide Cations, XCN’ (X = CI, Br, I): i 2 2 + g2nand B2rI ---* Band Systems
k2n
Jan Fulara, Dieter Klapstein,+Robert Kuhn, and John P. Maier* Institut f u r Physikalische Chemie, Uniuersitat Basel, CH-4056 Basel, Switzerland (Received: April 5, 1985)
-
-
The A22+ R211nand B2nn R2nQ (52 = 3/2, emission band systems of rotationally cooled chloro-, bromo-, and iodocyanide cations have been obtained by electron impact excitation of seeded helium supersonic free jets. The narrowing of the vibronic bands and particularly the resolution of the transitions of the individual isotopic species enable a vibrational analysis of most ?f the spectral features to be made. This leads to almost_allthe vibrational frequencies ( 1 2 cm-I) of these cations in their X211 states, as well as to many_valuesin the A22+ and B211 states. The spin-orbit splittings in the X211 states are also obtained, for X = C1 also in the B211 state, and better values for the higher ionization energies are given by combining the data of the present emission spectra with those from photoelectron spectroscopy.
Introduction The recent times have witnessed an upsurge in activity in the spectroscopic and reaction studies of ions.Ig2 This is associated on the one hand with the realization of the importance of ions in various planetary and extraterrestrial environments and phenomena and on the other hand with technological developments, especially in the laser field, enabling a variety of approaches to be exploited. As far as polyatomic cations are concerned, the earliest spectroscopic studies were based on emission experiments in disc h a r g e ~then , ~ in effusive sources excited by controlled electron impact: and more recently in supersonic sources.s The technique of laser-induced fluorescence has also been applied in the meantime to study rotationally cooled cations prepared by different means.2,6 Very high resolution measurements have been accomplished using either fast ion beam or laser beam arrangements,’ especially for predissociating ions,* and most recently using I R lasers and modulation techniques to record vibration-rotation bands of cation^.^,'^ Another approach, but at lower resolution, has been based on trapping the ions in rare gas matrices in conjunction with laser-induced fluorescencet1 and direct absorption techniques.I2 In this article we present and discuss the analysis of the emission spectra of halocyanide cations produced rotationally cold in supersonic free jets by electron impact excitation. Although the halocyanide cations, X-C=N+ (X = C1, Br, I), were among the first new cations for which the radiative relaxation of their lower excited electronic states was established by both electron impact excitationI3 and photoion-photon coincidenceI4 methods, the vibrational analysis of the emission spectra could not be accomplished at that time. The emi-%ion spec_tra were shown to-consist of two electronic transitions, A22+ XzII and B Z l l XzII, by comparison with the photoelectron spectra of these h a l ~ y a n i d e s . ’ ~However, only the two origin bands of the A2Z+ XzIIq (Q = 312, systems clearly stood 0 ~ t . ISub~ sequently, the BZII XzII transition of bromocyanide cation was investigated by trapping the ions in an ion cyclotron resonance
-
-
-
-
Prescnt address: Department of Chemistry, St. Francis Xavier University, Antigonish, Nova Scotia, Canada B2G 1CO.
0022-3654/85/2089-42 13$01.50/0
cell and recording the laser-induced fluorescence,but only a partial vibrational analysis was proposed.I6 (1) ‘Molecular Ions”; Berkowitz, J., Groeneveld, K.-O., Eds.; Plenum Press: London, 1983. “Molecular Ions: Spectroscopy, Structure and Chemistry”; Miller, T. A., Bondybey, V. E., Eds.; North-Holland Publishing Co.: Amsterdam, 1983. ‘Ionic Processes in the Gas Phase”; Almoster-Ferreira, M. A., Ed.; D. Reidel: Dordrecht, The Netherlands, 1984. “Gas-Phase Ion Chemistry”; Bowers, M. T., Ed.; Academic Press: London, 1984; Vol. 3. (2) Miller, T. A. Annu. Rev. Phys. Chem. 1982, 33, 257. Miller, T. A,; Bondybey, V. E. Philos. Trans. R. SOC.London, A 1982, 307, 617. Miller, T. A.; Bondybey, V. E. Appl. Spectrosc. Rev. 1982, 18, 105. (3) Herzberg, G. Q.Reo. Chem. SOC.1971, 25, 201. Leach, S . In “The Spectroscopy of the Excited State”; Plenum Press: New York, 1976 and references therein. (4) Maier, J. P.In “Kinetics of Ion-Molecule Reactions”; Ausloos, P., Ed.; Plenum Press: New York, 1979; p 437. Maier, J. P. Chimia 1980,34,219. Maier, J. P. Acc. Chem. Res. 1982, 15, 18. (5) Carrington, A.; Tuckett, R. P. Chem. Phys. Lett. 1980, 74, 19. Miller, T. A.; Zegarski, B. R.; Sears, T. J.; Bondybey, V. E. J. Phys. Chem. 1980, 84, 3154. Klapstein, D.; Leutwyler, S.; Maier, J. P. Chem. Phys. Lett. 1981, 84, 534. (6) Miller, T. A,; Bondybey, V. E. J. Chim. Phys. Phys.-Chim. Bioi. 1980, 77, 695. Lester, M. I.; Zegarski, B. R.; Miller, T. A. J. Phys. Chem. 1983, 87, 5228. (7) Carrington, A. Proc. R. SOC.London, A 1979, 367,433. Carrington, A.; Softley, T. P. In “Molecular Ions: Spectroscopy, Structure and Chemistry”; Miller, T. A,, Bondybey, V. E., Eds.; North-Holland Publishing Co.: New York, 1983; p 49. (8) Edwards, C. P.; Maclean, C. S.; Sarre, P. J. Chem. Phys. Lett. 1982, 87, 11. Abed, S.; Brover, M.; Carre. M.; Gaillard, M. L.; Larzillitre, M. Chem. Phys. 1983, 74;97. (9) Oka, T. Phys. Rev. Lett. 1980, 43, 531. Schafer, E.; Saykally, R. J. J. Chem. Phys. 1984,80,2973. Altman, R. S.; Crofton, M. W.; Oka, T. J. Chem. Phys. 1984, 80, 391 1 and references therein. (10) For a recent review see: Gudeman, C. S.; Saykally, R. J. Annu. Rev. Phys. Chem. 1984, 35, 387. (11) Bondybey, V. E.; Brus, L. E. Ado. Chem. Phys. 1980, 41, 269. Bondybey, V. E.; Miller, T. A. In “Molecular Ions: Spectroscopy, Structure and Chemistry“; Miller, T. A,, Bondybey, V. E., Eds.; North-Holland Publishing Co.: Amsterdam, 1983; p 125. (12) Andrews, L. Annu. Reo. Phys. Chem. 1979,30,79. Bondybey, V. E.; Miller, T. A,; English, J. H. J . Chem. Phys. 1980, 72, 2193. (13) Allan, M.; Maier, J. P. Chem. Phys. Lett. 1976, 41, 231 (14) Eland, J. H. D.; Devoret, M.; Leach, S. Chem. Phys. Lett. 1976,43, 97. (15) Heilbronner, E.; Hornung, V.; Muszkat, K . A. Helu. Chim. Acta 1970, 53, 347. Lake, R. F.; Thompson, H. Proc. R. SOC.London, A 1970, 317, 187.
0 1985 American Chemical Society
4214
The Journal of Physical Chemistry, Vol. 89, No. 20, 1985
Fulara et al.
'
I
I
I
11800
I
1; I
IO8
-
11600
I
I
23000
22000
I
11400 V(crn-9
Figure 1. The main part of the A2Z* R2110 ( a = 3 / 2 , emission band system of chlorocyanide cation obtained by -200-eV electron impact excitation on a seeded helium supersonic free jet. The optical resolution was 0.12 nm (fwhm). Some of the vibrational assignments are system. The conindicated, and a horizontal bar denotes the R = secutively numbered vertical markers, intermittently labeled with the circled numbers, reference the bands listed in Table I. Atomic lines (He, Cl) are marked with a dot.
It has now proved possible to vibrationally assign most of the stronger bands in the emission spectra of rotationally cold halocyanide cations. These spectra have been obtained by electron impact excitation of their molecular precursors seeded in a helium supersonic free jet. The main benefit of this approach in the present case is, due to the narrowing of the vibronic bands, that the isotopic splittings of the naturally occurring 35Cl,37Cland 79Br, slBr species become discernible. This enables the vibrational progressions to be identified. In addition, the sharpness of the bands enables the vibrational intervals to be determined to within f 2 cm-I (or better) as opposed to f 1 0 cm-I when effusive sources are used. This also aids the analysis. Nevertheless, the emission spectra are still complex bectuse of the large geometry change taking place during the B211 X211 electronic transition, and there are numerous weaker bands which are not assigned. Part of this complexity is associated wiih vibraticnal redistribution as a result of coupling between the A2Z+ and B211 states. This is indicated by the lifetimeI3 photoionand photoelectron-photon'8 coincidence measurements, which show this in the variation of the fluorescence quantum yields and the pronounced nonexponential decay curves. Another set of data relevant to the present studies are these from the_ measurements of the absorption spectra of the B2113,2 X2113,z transition of the halocyanide cations in 4.5 K neon matrices." The complete relaxation of the ions to the lowest level of the X211state enables the 52 = 3 / 2 origin bands to be located in the matrix and hence their expected region in the gas-phase emission spectra to be estimated.
-
I
11
I
I
21000
1s I
I
-
I
I
19000
20000
18000 T(crn-') Figure 2. A portion of the B211n (R = 3 / 2 , emission band system of supersonically cooled chlorocyanide cation recorded with 0.04-nm resolution. Further details as given in caption of Figure 1.
TABLE I: Maxima of the Prominent Bands in the 2 i t ' +%nn Emission Spectrum of Chlorocyanide Cationa (n= 3/2,
label 1
2
Vvac/cm-'
assign t
11 858 11 855 11 845 11 842 11 691 11689
0:
11 640
1:
2:
3 4
::;:;} 11 562
0:
+-
Experimental Section The emission spectra of the halocyanide cations were recorded on the apparatus describ&i.20 Briefly, the respective halocyanide, at a pressure of 100 mbar, was mixed with excess helium (- 1.4 bar) and was expanded through a 100-pm nozzle into a collision chamber. The resulting background pressure was mbar. The supersonic free jet thus formed was intersected with a magnetically constrained electron beam (3-6-mA current, 200-eV energy) 5 mm downstream from the nozzle. Emitted photons were dispersed by an fl9.5, 1.26-m monochromator with resolution in the range 0.03-0.12 nm (fwhm). The spectra were
-
-
(16) Grieman, F. J.; Mahan, B. H.; OKeefe, A. 0.J . Chem. Phys. 1981, 74, 857. (17) Castelluci, E.; Braitbart, 0.;Dujardin, G.; Leach, S. Faraday Discuss. Chem. Soc. 1983, No. 75, 90. (18) Maier, J. P.; Ochsner, M.; Thommen, F. Faraday Discuss. Chem. SOC.1983, No. 75, 7 7 . (19) Leutwyler, S.; Maier, J. P.; Spittel, U. J . Chem. Phys., in press. (20) Klapstein, D.; Maier, J. P.; Misev, L. In "Molecular Ions: Spectroscopy, Structure and Chemistry"; Miller, T. A,, Bondybey. V. E.. Eds.: North-Holland Publishing Co.: New York, 1983; p 175.
5 6
l11 l4 413 l5I
3631
-
1: 11 357 a The bracketed bands correspond to the individual isotopic species and/or to multiplet vibronic structure. The SZ = bands are distinguished by the horizontal bar. The labels refer to those in Figure 1. All values k 1 crn-'. l1 11 361
7
recorded by use of single photon counting electronics and an on-line LSI 1 1/03 microcomputer. The numerous helium emission lines apparent in the spectra were used for the wavelength calibration.
Results and Discussion Chlorocyanide Cation. I? the emi2sion spectrum of chlorocyanide cation the A2Z+ X211 and B211 X211 band systems are well separated. The main parts of each of these systems are presented in Figures 1 and 2, respectively. Where_asthe lo_ngwavelength transition exhibits only a few'bands, the B211 X211 transition consists of an extensive collection of bands. This difference reflects the relatively small geometry changes accompanying the former transition but large ones for the latter. The actual bond length changes have been evaluated from the photoelectron spectrum of chlorocyanide in conjunction with Franck-Condon factor calculations, which show that the dominant change is an increase (by -0.2 A with res ect to the molecular distance) of the C-CI bond length in the b2nstate of the cation.*'
-
-
-
Supersonically Cooled Halocyanide Cations
The Journal of Physical Chemistry, Vol. 89, No. 20, 1985 4215
XABLE 11: Maxima of the Prominent or Assigned Bands in the Bzna -+Kana(a= 3/z, 1 / 2 ) Emission Spectrum of Chlorocyanide Cationa label
~v,c.cm-f
1 2
23 574 23 428 23 336 23268 23260 I 23 048 22 833 22 819 22 745 22 664 22 596 22513 22 445 22 307 22 245 22 220 22 147 22 006
3 4 5
6 7 8 9 10 11
12 13 14 15 16 17 18
21 923 19 21 745 757
20 21695 21 688
21 22 23 24
21 654 21 21495 480 21 421 21 409 397
25 26 27 28 29 30 31
21 122 104 20 930 20881 20 830
label ~vvac/cm-' assignt 37 20 096 20107 38 39 40
42 43 44 45
47 48 49
t
51 52 53
t
54 55
i 20 20598 579 t 20 533
I
2o022 20 01 1
I
19 19855 833
t
19788 19 765
I
19745 19 737
t
1 19 551 l9 19571 584
i l19 9294 277 t
56 57 58 59 60 61
19 233 19 196
t 18976 18 952 1 18 910 19 004 19027
18 809 18 766 752
~~
electronic state XlZ+
Pn
52
VI
v2
v3
-A
3i2
729 823
397
2201 1916 1914
276f 2
t
19 l9 498 485
46
50
1
20 048 2o068
41
I
21 345 21 182
20 20677 657 32
i
assignt
TABLE III: Vibrational Frequencies (h2 cm-I) of Chlorocyanide Cation in the Ground and First Two Excited Electronic States"
t
I 18671 l8 689 t 18 527 18 743 l8 730
18 420
t 18 166 139 t 17 967 l 7 860 17 855 t 17 807
18 247 210
62 33 63 17 756 34 20 393 64 17 607 35 20 358 17 571 65 36 20 304 Bands with a resolved chlorine isotope pattern are coupled by braces. The C2 = system assignments are denoted by a horizontal bar above the transition. The labels refer to the markers in Figure 2. All values i 1 cm-'.
-
In Takle I are gi_venthe wavenumbers of the few stronger bands X211 transition. The spectrum itself suffers of the A22+ somewhat in quality (cf. Figure 1) because of the low sensitivity of the photomultiplier in this region. The two origin bands clearly stand out, and tbeir separation, 276 f 2 cm-', is the spin-orbit splitting in the X211 state of chlorocyanide cation. The other assignments given in Table I are tentative; it is supposed that 1; and 2; transitions for both spin-orbit components are observed and that the 21 transition may be split by Renner-Teller interactions into the two cdmponents. The limited spectral details/ qualitygreclud_e a further discussion. The B211 X211emission band system is complex and extensive in the 400-600-nm region. The short-wavelength part of the spectrum is in addition overlapped by the violet emission bands of the BZZ+ X22+ transition of the C N radical. A consistent vibrational analysis of the prominent emission bands was achieved, based on the trends of isotope splittings due to the 35Cl-C==N+ and 37CI-C=N+ species in the respective
-
-
(21) Hollas, J. M.; Sutherley, T. A. Mol. Phys. 1971, 22, 213.
363 i 3 "The frequencies given are for the 35C1isotopic species. The ground molecular values are taken from ref 22. The last column shows the inferred spin-orbit splittings.
progressions and combination series, as well as the vibrational intervals themselves. Both spin-orbit components of the transition, Le. %3/2 'n3/2 and 2rI~/2 2r11/2,were identified, and the SZ = ' / 2 bands are designated (as is the case for the other halocyanides) in the figures and tables by a horizontal bar above the vibrational mode excited. The latter are numbered according to Herzberg; vi and u3 are the C-CI and CEN stretching fundamentals of 2' symmetry, and v2 is the II symmetry bending vibration.22 The same notation is used for the bromo- and iodocyanide cations. The wavenumbtrs of the maxima of the assigned, or more intense, bands in the B211 X211emission system of chlorocyanide cations are collected in Table I1 together with their vibrational assignments. In making the vibrational analysis, two possible interpretations arose: either that the origin band of the B2113/2 XZn3/2 transition is at 22 220 f 1 cm-' (as indicated in Figure 2) or that it is the band lying 525 cm-' to the blue (band 8 in Figure 2). The progression and combination series can be built up from either of these two origins with reasonable isotopic splittings. However, in the latter interpretation many of the stronger bands would have to be assigned to transitions involving the excitation of the degenerate v2 mode in double quanta, starting with band 15 being 2;. The preferred choice is thus for band 15 to be the origin as most of the stronger bands are then assigned, as indicated in Figure 2 and Table 11, to transitions associated with the totally symmetric v i and v3 modes. The origin band of the R = subsystem (@) is located at 22 307 f 1 cm-', and thus _the difference bttween the spin-orbit splittings in the B211 and X211 states, A,'(B211) - A{'(X211),is -87 f 2 cm-', the states being inverted. Because A,,"(R211) = -276 f-2 cm-' according to the separation of hte!: and @ bands in the A2Z+ R211 transition (Figure I), A,'(B2n) = -363 f 3 cm-'. The positions of the origin bands in the emission spectrum (Figures 1 and 2) can also be used to obtain better values fo_r the adiabatic ionization energies (0 leading to the X211i/2and B211, (0 = 3/2, states than is possible from the photoelectron spectrum of chlorocyanide alone. Combining the emission data with I(_X2113/2)= 12.34 f 0.01 eVZ3leads to I(X211112)= 12.37 eV, I(B2113/2) = 15.09 eV, and 1(B2111,,) = 15.14 eV,_the error of all the values being fO.O1 eV. The I values for the B211state are lower by -0.04 eV compared to those determined from the photoelectron spectrum,'' and show that in the latter spectrum the first discernible peak in the band associated with the B211state does not correspond to the adiabatic ionization transition but to the one leaving the ion in the first level of the v 1 vibration within the il = 3/2 component. The inferred A,' and AO" values are included in Table 111. The B211 X211 band system is dominated by progressions and combination series involving the vl", C-C1 stretching mode, in the ground cationic state, in accord with the main geometry change being in the C-CI distance. The various emission bands show a characteristic isotope splitting, e.g. 12 cm-' for I:, and this increases monotonically with further excitation, e.g. 19 cm-' for 1: or 23 cm-' for 1; (see Table 11). These values agree rea-
-
-
-
-
-
-
(22) Herzberg, G. "Molecular Spectra and Molecular Structure"; Van Nostrand: Princeton, NJ, 1968; Vol. 11. (23) Dibeler, V. H.; Liston, S. K. J . Chem. Phys. 1967, 47, 4548.
4216
The Journal of Physical Chemistry, Vol. 89, No. 20, 1985
Fulara et al.
TABLE V: Maxima of the Prominent, or Assigned, Bands in the Ban3,, -+f['II,,, Emission Spectrum of Bromocyanide Cationa
label '&ac/cm-l 1 21 21 461 448
2 3 4 5 I
I
1
I
I
14100
13900
13700
13500
13300
I
6 7
8
21 21 391 380 21 173 20 995 20 986 20 938 20 929
9 10
1
I
I
I
12300
I
-
12100
13
?(ern-1)
Figure 3. Main part of the AZZ* RzIIn ( Q = 3/2, emission band system of bromocyanide cation in a supersonic free jet. The optical resolution was 0.04 nm. Further details are as given in legend of Figure
14 15
1.
xz
16
TABLE 1V: Maxima of the Assigned Bands in the E' -+%nn (a= 3 / , , Emission Spectrum of Bromocyanide Cationa
label 1
2
3 4 5 6 7 8 9 10
11 12
13
14 15 16 17
-v,,,/cm-
14 120 14118 13 833 13 829 13 827 13 824 13 13723 719 13 699 13696
1
l1 I 1 729 725
22 3:
I
0:
1: 2;
2: 1: 2:
1
2; -
2;
I
1
23 24
1: 1:
I
1 1 11924 11 921 1
11 790 l1 793
2;
t
t 13 543 I 13546 13411 13 408 I l13 3 140 137 12 639 12 346 12 344 12 340 12 335 12 222 12219 12 144 12 141
18 19 20 21
1 3 630 633 13 13 555
13 329 13333
17
assignt
0:
1: -
2: 3: 1: 3;
a The braces join individual isotopic species and/or multiplet structure. The labels refer to Figure 3, and the = transitions are marked by the horizontal bar. All values t 1 cm-'.
sonably with calculated splittings from a b initio procedures, 9, 18, and 26 cm-I for the above three transitions, respectively.24 The
25
31
20 480 2o 470
1
33
2o 20 376 371
I
34
l8 400 18 403
\
18 379
304t
l18 8 300 18 015 1 7 932
35 36 37
t 2o 20 01 0221 t 19 831 19 822 1 l 9 729 19 719 1 20 l177 2o g3
I
11900
I 18 l8 514 521 t 18 487 l8 483 I l 8583 18 584
30
32
1
18 610 18 6041
29
20 192 12500
18 672 l8 675
28
1 1
assignt
18 840 18 832) 18 830
27
I
11 12
label TvaC/cm-' 26
207301 20 717 2o 688 697 I 20 2o 560 20 556
x3
assignt
t 17 l 7 712 715 1 l17 7 863 860
38 39
6351 17 460 468 I 17 363 17 274 2791 17 185 17 l 7 631 17 543
40 41
l7
1 l 9 631 19 637 I l 9 705 19 699
42 43
19 552 19 529
44 45 46 47 48
l7
1 l 9 363 19 373 t l 9 502 19 492
19 230 192341
t 19 063 t
19 l 9 146 141
16 890 16 760 16 742 16 16631 630
49 16 l6 539 534 16 241
50
t
1
l9
19 19056 051
1
The braces couple resolved bromine isotope bands. bering labels correspond to those in Figure 4.- All values-+l cm-'. a
beginning of the vl' progression in the excited state is also apparent, though the higher members are obscured by the strong CN emission bands. The fundamental vibrational frequencies of the u 1 mode are 823 f 2 cm-I in the XzI13j2state and 525 f 2 cm-' in the B2113,2state (Table 111). In the ground state the excitation of the v3, C=N stretching mode is also evident, the fundamental frequency being 1916 f 2 cm-I. All the dedused vibrational data are given in TableJII. The u1 value for the AZZ+state is obtained from u,: in the X2n state and the position of the 1 sequence band in the A%+ X211 system (Table I). For comparison, the fundamentals of the neutral molecule are also included in Table 111. As can be seen, the largest difference in the frequencies is for the v , mode connected with the respective geometry changes mentioned earlier. Bromocyanide Cation. Figures 3 and 4 show the emission spectra of rotationally F l e d brom_ocyanide-cation which correXzII and BZII X2n transitions, respond to the A2Z+ spectively. Whereas the latter s p t e m is composed of many bands in the 460-620-nm region, the A2Z+ X z n system only shows a few bands with dominant 0; and @ transitions. In Tables IV and V the wavenumbers of the maxima of the stronger bands in
-
-
-
-
(24) Botschwina, P., private communication.
Supersonically Cooled Halocyanide Cations
The Journal of Physical Chemistry, Vol. 89, No. 20, 1985 4217
1;z;
TABLE VI: Vibrational Frequencies (A2 cm-l) of Bromocyanide Cation in the Ground and First Two Excited Electronic Statesa
1;2;
I
I
I
electronic state
uI
v2
X’Z+
580
gn3,2
650 584 47 1
368 288 421 394
A22+ fi2n3/2
-A 2187 1906 1930 1939
1477
‘The frequencies given are for the 79Br isotopic species. The ground molecular values are taken from ref 22. The last column shows the inferred spin-orbit splitting ( h 2 cm-I). I
I
I
21000
20000
I
I
-
19000
I
I
S(cm-1)
17000
18000
Figure 4. The main part of the B2n3/2
A2n3/2emission
band system of supersonically cooled bromocyanide cation. The optical resolution was 0.03 nm. Further details as for Figure 1.
-
the emissicn spectra and the proposed assignments are presented. In the A28+ X211transition (Figure 3), the positions of the 0; and @ bands provide the spin-orbit splitting for the lowest vibrational level of the X211state, Le. A{ = -1477 f 2 cm-’. The double-headed appearance of the origin bands is a result of the bromine isotope splitting as well as the rotational profiles. The proposed assignment of the weaker bands is based on the following grounds. The 2; transition bands have a quartet structure; two components correspond to the bromine isotopes, and another two are presumed to be 2 , A components obtained as result of Renner-Teller interactions. In addition, the distinct bands -290-300 cm-] to lower energy of the origin bands can only be assigned to the 27 and !$ transitions and there are weak bands at the expected and 2; transitions (cf. Table IV). The positions for the 2;, nearest band to red of the origin bands is associated with the 1; and transitions because the v 1 frequency-in the R211 state is known to be 650 f 2 cm-’ from the B211 XzII transjtion (vide infra), and electronic structure arguments point to an AZZ+state value very similar to that in the molecule (Le. -580 cm-I). The opposite trend is expected for the v3 vibra_tionalfrequency: a larger decrease in the X211 compared to the AZZ+state relative to the molecular value. The weak band -30 cm-I to the blue of the 0; band may be the 31 transition. The 37 band can also be identified (cf. Figure 3), but the counterpart is not detected since it lies outside the wavelength region accessible in the experiment. The vibrational interpretation of the B211 R211band system became possible mainly because the individual isotopic transitions are resolved in the present spectrum (Figure 4). The changes of the isotopic splittings along progression and combination series can be followed and the_assignmenis corroborated. All the bands assigned belong to the B2113/2 X2113/z subcomponent system, and the origin band is found at 19 234 f 1 and 19 230 f 1 cm-’ for the-79Br-C=N+ and 8’Br-C=N+ species, respectively. The B211 R211emission spectrum comprises long and strong progression and combination series involving the excitation of the v l (C-Br stretching) vibration in both the states. The isotope splitting in the 1; progression first decreases, from 4.3 cm-I for the 0; band to a minimum at m = 2, and then steadily increases (Table V). In the case of the 1: pFogression the splitting increases monotonically. These observations indicate that the isotopic separation of the zero-point levels in the B211 state is larger by
2,
-
2
-
-
-
4.3 cm-I than in the R2II state. As the splitting increases with vibrational quantum of the v 1 vibration, for the 1; transition the separations appear comparable in the two states. Thus, for the Ot, 17, and all 12, transitions, the 79Brband lies to higher energy of the *IBr one, whereas for 11 ( m > 2) it is the opposite. Besides the v 1 progressions, the excitation of the v3, CTN stretching mode is evident (also in combination series) in the B211 state. Moreoever, the v2 bending vibration is excited in double quanta in both the excited and ground states and appears strong in the combinations with the v 1 mode. All the inferred vibrational frequencies are collected in Table VI. For comparison, the observed vibrational frequencies in the B2113/2 X2113/z neon matrix absorption spectrum at 4.5 K are as follows (A10 cm-I): vl’, 478; vZ’, 377; v3’, 1830 cm-l.19 A part of the fluorescence excitation spectrum of the B211 R2II transition of bromocyanide cation (in the 20000-25 OOO-cm-l region) has been reported by using laser excitation of ions confined in a quadrupole trap.I6 However, the spectrum is complex and the bands are broad due to the high temperature of the ions formed by electron impact. Although a partial vibrational analysis was proposed, by reference to the present assignments (Table V) it is seen that most of that interpretation is not correct. While the approximate value given for vl’ as 441 f 18 cm-’ is of the right magnitude, the vl” frequency deduced (509 f 16 cm-I) is wrong, vl” = 650 f 2 cm-I (Table VI), as is the difference between the spin-orbit splittings ( A ” - A’ = -224 f 43 cm-]).l6 Although none of the bands belonging to the R = subcomponent system could be identified in the present emission spectrum, a reasonable estimate for A’ can be made based on the observed spin-orbit splittings in bromo-substituted speciesz5 The sum of the spinorbit splittings, which @ the case of bromocyanide cation is distributed between the X211 and B211 states, will be around 0.30 eV (Le. within the limits 2400 f 200_cm-’). Because A / = -1477 f 2 cm-I according to the A2Z+ X211transition, it follows that A,,’ -900 f 200 cm-I and hence A” - A’ -600 f 200 cm-I. This in turn indicates that the origin band of the B2111/2 %’II,I~ system should lie between 18 400 and 18 800 cm-I. The strongest band of the latter system is expected to be that of the transition and thus would correspond to one of the weaker bands apparent in the spectrum between the 1; and 172: bands. The reason for subsystem is not clear. the weakness of the R = Zodocyanide Cation. The observed emission spectrum of rotationally cold iodocyanide cation is shown in Figures 5 and 6. subIt comprises the two well-separated R = 3 / 2 and R = systems of the AZZ+-* X211QtranGtion, bet_ween which are found a few bands associated with the BzII X211 system. The A2Z+ R2II band system shows, as in the case of the other halocyanide cations, dominant origin bands, but in addition intense bands associated with the 27 and transitions. The remaining bands are weaker and are assigned as indicated in Table VII. In the excited A2Z+ state, only the vz vibration is excited, whereas the g2rIstate v2 and v I appear to be excited. The excitation of the degenerate v2 mode by a single quantum is allowed if Renner-Teller interaction i s includ_ed. The observed trend in the emission spectra of the A22+ X211 transition of the halocyanide cations, XCN+, is that the intensity of the 2: and transitions increases for X = C1 to X = I relative to the origin bands; at the
-
-
-
-
-
+
rf
-
-
-
3
( 2 5 ) Heilbronner, E.; Hornung, V.; Maier, J. P.; Kloster-Jensen, E. J . Am. Chem. SOC.1974, 96, 4253.
4218 The Journal of Physical Chemistry, Vol. 89, No. 20, 1985
TABLE VII: Maxima of the Prominent, or Assigned, Bands in the Emission Spectrum of Supersonically Cooled Iodocyanide Cation’” label Tvac/cm-’ assignt label T,,,,/cm-’ assignt
I
log
Fulara et al.
29 I
1
I
I
18500
1
18000
7 8 9 10
1
17500
11
12 13 14 15 16 17 18 19
x5
I
-
14500
,
14000
Figure 5. The A2Z+ g2nn ( Q = 3/2, emission spectrum of iodocyanide cation in a supersonic free jet recorded with 0.06-nm band-pass. Further details as in Figure 1 legend. Note that, energetically, the spectrum shown in Figure 6 fits in between the upper and lower traces shown above.
17
1; I 1:
t
32 33 34 35 36 37
R2n
‘12
470 535 559
If2
473
3/2
A2Z+ B2n,,2
I
I
16500
-
I
I
-
E
-
-
-
-
1: B
321 239 253 274
2158 2082 4343
-
Skm-1)
16000
same time the inten_sityof t_he 1; bands decreases. In the case of the B211 X2H transition, only bands belonging subsystem are identified. The origin band is to the R = located a t 16 173 f 1 cm-I (Figure 6), and a progression in ul’, cpd sequence. transitions, are observed. In Table VI1 the B211 X211-transitio_nsare distinguished by the letter B from those of the A2Z+ X211ones. The frequency of the u1 mode obtained for the w2111/2state is 559 f 2 cm-’ and 473 f 2-cm-I in the 82111/2 state (Table VIII). The v1 value for the B2113,2. state inferred from the absorption spectrum in a neon matrix is 400 f 20 cm-l.I9 The difference between the frequencies of the u , mode in the i2 = 3 / 2 and l/z subcomponents is also seen in the photoelectron spectrum of iodoacetylene,15 where although the absolute error in the vibrational intervals is large f40 cm-I, comparison of the relative spacings clearly shows that uI (Q = > u1 (i2 = 3/2). The assignmect of the respective bands to either the A2Zf g211or BzII X211transitions is supported by the following: (i) the R = 3 / 2 and subsystems of the former transition are practically superimposable (cf. Figures 5 and-6), and (ii) the different lifetime characteristics of the A22+and B211states enable the band systems to be separated by recording time-resolved emission spectra. This had been carried out previously by using an effusive source,’’ and by comparison, the bands can be again identified in the present spect_ra. Bands due to the B2113/2 X2113/2subsystem are not obse-rved. Thejocation of the origin band_can be estimated from the A22+ X211 transition, i.e. A” (X) = -4343 f 2 cm-I, and the
-
1; B
“The ground molecular values are taken from ref 22. The last column shows the determined spin-orbit splitting (k2 cm-I).
Figure 6. A portion of the BzIIl/2 Z%2111/2 emission spectrum of supersonically cooled icdocyanide cation. The optical resolution was 0.06 nm. Further details as given for Figure 1.
-
-
’” The bands belonging to the E2n112 +,f72n112 transitions are Gentifiekwith the letter B in the assignment, whereas in the A2Z+ +X2na transition the horizontal bar above distinguishes system. The label numbering refer to the markers in the = Figures 5 and 6. All values t 1 cm-’.
X’Z+
17000
16646 16 575 16 283 16 180 16 173 16 089 16011 15 614 15 533 15 423 14 201 1 3 946 139391 13919 13 687 13 666 13433 13 366 13196
TABLE VIII: Vibrational Frequencies (f2 cm-’) of Iodocyanide Cation in the Ground and First Two Excited Electronic States‘ electronic state Q VI v2 v1 -A
I1 ’
I
18 294 307 18 262 18 069 18023 18 001 17 805 17 727 17 636 17 602 17 580 17 415 17 460 17 211 17 128 17 027 16 742
20 21 22 23 24 25 26 27 28 29 30 31
V(crn-1)
13500
-
18618 18563 18536
“sum-rule” e~pectation,~’ A” + A’ -5240 f 160 cm-’ or A’ -890 f 160 cm-l. Thus, the 0; band of the i2 = 3/2 subsystem is expected to higher energy of the band, around 19630 f 160 cm-I. This value agrees within the error limits to the difference +tween the given adiabatic iocization energies leading to the X2113/2 (10.87 f 0.02 eV)23and B2113/2(13.31 f 0.02 eV)26states. In the N e matrix, the origin band of the B2113/2 e R2113j2absorption spectrum is found at 20023 f 16 cm-I.l9 However, no distinct bands are detected in this energy region i,n the emission spectrum. The explanation offered is that the B2IIU2state is depleted by a rapid nonradiative transition involving the A2Z+ state. This is favored because the lowest level of the A2Z+state lies merely 0.14 f 0.02 eV below ‘1energy. In contrast, the gap is larger to the lowest level of the B2111j2state, -0.25 eV. Similar observations have been made on the relaxation behavior of halocyanoa~etyIene,2~ and other related cations,28where a small energy gap between two electronic states results in the absence of the expected emission bands. Radiative relaxation does finally take place, as the photoelectron-photon coincidence measurements show, but the fluorescence is red-shifted and is unstructured, due to the redistribution of the internal vibrational energy. An analogous situation could prevail in the case of iodocyanide cation. Concluding Remarks. A vibrational analysis of the A2Zi R211 and B211 X211 emission band systems of the halocyanide
-
-
-
~~
~
~
(26) Unpublished data from this laboratory (27) Kuhn, R.; Maier, J. P.; Thommen, F. J . Electron Specrrosc. Relat. Phenom. 1984, 34, 253. (28) Maier, J. P.; Misev, L.; Thommen, F. J . Phys. Chem. 1982,86, 514. Maier, J. P.; Thommen, F. In “Gas-PhaseIon Chemistry”; Bowers, M. T., Ed.; Academic Press New York, 1984; Vol. 3, p 357 and references therein.
J. Phys. Chem. 1985, 89, 4219-4225 cations has become possible because of the darrowing of the bands, and especially due to the resolution of the bands of the naturally occurring isotopic species by using a supersonic source. Thus, vibrational frequencies (to within *2 cm-I) could be obtained for almost all the fundamectals in the-ground cationic state, as well as many values in the AZB+and BZII states. These values are summarized in Tables 111, VI, and VIII. This spectroscopic information on these small polyatomic cations should be valuable for comparison with theoretical approaches, in the determination of force constants, and as the starting point in very high resolution studies of ions such as those becoming prevalent in the I R region .9J0 As far as the changes of the cationic vibrational frequencies, with respect to the molecular values, reflect the molecular orbital description of the electronic structure, this has already been amply discussed in the various photoelectron s t ~ d i e s . ’The ~ ~ ~present ~
4219
study does however yield many more vibrational frequencies and of course much more precisely. Also along these lines, the spin-orbit splittings in the ground cationic states are directly obtained from the A%+ X211 transitions, and by combination of the emission data on the BzII X211 transition with the photoelectron data, better values for the higher ionization energies and spin-orbit splittings in the B211 state can be obtained.
- -
Acknowledgment. This work is part of Project No. 2.429-0.84 of the Schweizerischer Nationalfonds zur Forderung der wissenschaftlichen Forschung. Ciba-Geigy SA, Sandoz, SA, and F. Hoffmann-La Roche & Cie. SA, Basel, are also thanked for financial support. We thank Dr. S. Leutwyler for his help in the recording of a part of a n early spectrum of BrCN+. Registry No. CICN+, 37612-72-9; BrCN+, 34749-77-4; ICN*, 34749-78-5.
Range and Range Straggling of Low-Energy Electrons In the Rare Gases‘ Jay A. LaVerne and A. Mozumder* Department of Chemistry and Radiation Laboratory, University of Notre Dame, Notre Dame, Indiana 46556 (Received: April 8, 1985)
Stopping power and inelastic mean free path of low energy electrons have been consistently calculated for the rare gases down to twice the ionization potential following our earlier procedure ( J . Phys. Chem. 1985,89, 930). The chief ingredient of the calculation, viz., the oscillator strength distribution, is obtained from literature values and compilations. Range distribution has been calculated by our previously developed theory using the collision-by-collision approach. From this distribution mean and median ranges are obtained and these are compared with the CSDA range computed by integration of the inverse stopping power. Relative width and skewness of the range distribution have also been calculated as functions of incident energy. Comparisons with experiments and other calculations have been provided wherever possible.
Introduction It is important to understand the interaction of low-energy electrons with matter because a substantial amount of energy deposited by all ionizing radiation leads to their production. Also, the geometry of spurs, Le., the localized regions of radiation chemical reactions, as well as that of heavy particle tracks are to a large extent determined by low-energy electrons2 The rare gases make up only about 1% of our atmosphere, but they are very important in cosmic and solar radiation studies. The rare gases are also used extensively in ionization chambers where low-energy electrons may have a dominant role. The stopping power of matter for low-energy electrons has been difficult to determine since quantum-mechanical formulas such as Bethe’s3 are only suited for high energies because of the use of approximations in addition to the Born approximation. A major feature of Bethe’s theory is that it includes the energy-independent mean excitation potential which represents a logarithmic average of all energy loss processes to the medium. Some inner shell electrons, especially with the heavier elements, will never be energetically available to low-energy electrons! LaVerne and M o ~ u m d e rhave ~ . ~ previously described a method for determining
the stopping power of matter for low-energy electroris by combining a formula originally derived by Ashley’ with the oscillator strength distribution of the medium. From the latter, one can obtain the energy dependence of the stopping power parameters. Ashley employed the Born approximation and an exchange corrected cross section using the formalism of complex dielectric constant in which the binding energy is included in the first order. In the present paper the oscillator strength distributions of the rare gases have been compiled from experimental photoabsorption cross sections and the stopping powers were calculated for electrons up to 10 keV. The length of the crooked path of the electron or continuous slowing down approximation, CSDA, range was also calculated directly from the stopping power. Range straggling which is the statistical variation in path length of electrons with complete energy loss has*beenshown not to be Gaussian with low-energy electrons: Low-energy electrons suffer relatively few collisions over their range and so are not suitable to the range straggling methods of Bohrs and L a n d a ~ . ~Mozumder and Laverne6 have previously described a collision-bycollision approach to determine the range straggling along the crooked path of the electron. In this paper these calculations are extended to the rare gases.
(1) The research described herein was supported by the Office of Basic Energy Sciences of the Department of Energy. This is Document No. NDRL-2703 from the Notre Dame Radiation Laboratory. (2) Mozumder, A. Adu. Radiat. Chem. 1969, I , 1. (3) Bethe, H. A. Ann. Phys. 1930, 5 , 325. (4) Mozumder, A. In ‘Proceedings of the 3rd Tihany Symposium on Radiation Chemistry”; D o h , J., Hedvig, P.,Eds.; Akademia Kiado: Budapest, Hungary, 1972; Vol. 2, p 1123. (5) Laverne, J. A.; Mozumder, A. Radiar. Res. 1983, 96, 219.
(6) Mozumder, A,; Laverne, J. A. J . Phys. Chem. 1984,88,3926; 1985, 89, 930.
0022-3654/85/2089-4219$01.50/0
(7)
Ashley, J. C.; Williams, M. W. “Studies of the Interactions of Ionizing Radiations with Communications Materials”; Rome Air Development Center, Griffiss Air Force Base New York, 1983; Internal report RADC-TR-83-87, pp 5-10. (8) Bohr, N. Philos. Mag. 1915, 30, 581; K . Dan. Vidensk. Selsk., Mat.-Fys. Medd. 1948, 18, (8). (9) Landau, L. J . Phys. (USSR) 1944, 8, 201.
0 1985 American Chemical Society
