J. Phys. Chem. 1992,96, 1130-1141
1130
nuclei X,(g) with aging. The aging process could consist of the formation of agglomerations of dust particles, a single dust particle possibly not having a crevice to stabilize a gas pocket. The experiments of Figure 1 show once more that the inception of cavitation by ultrasound is decisively determined by the pre-
treatment of the solution.
Acknowledgment. We thank Dr. J. Lilie and Dr. E. Janata for technical advice. This work was supported by Deutsche Forschungsgemeinschaft und Fonds der Chemischen Industrie.
Negatlve Ion Photoelectron Spectroscopy of HCF-,, HCCr, HCBr-, and HCI‘: Photoelectron Angular Distrlbutlons and Neutral Triplet Excltation Energies Mary K. Gill- Kent M. Ervin,+ Joe Ho, and W. C. Lheberger* Joint Institute for Laboratory Astrophysics, University of Colorado and National Institute of Standards and Technology, and Department of Chemistry and Biochemistry, University of Colorado, Boulder, Colorado 80309-0440 (Received: August 2, 1991)
-
Photoelectron spectra and angular distributions are reported for the HCX(’A’) + e- HCX-(k 2A’’) and Hex(””) + e- HCX-(k A”) transitions of the halocarbenes (X = F, Cl, Br, and I). Taking photoelectron spectra at parallel and perpendicular laser polarizations with respect to the direction of the photoelectron detection allows us to distinguish the triplet transition from the overlapping singlet transition. Ab initio calculations used to simulate the Franck-Condon envelope for the triplet states combined with the experimental data predict that HCl has a triplet ground state. Best estimates for the triplet excitation energy based on these simulations are 14.9 f 0.4 kcal/mol (HCF), 4.2 f 2.5 kcal/mol (HCCl), 2.6 f 2.2 kcal/mol (HCBr), and -2 to -10 kcal/mol (HCI). Vibrational intervals of 850 f 60 cm-I (HCCI), 725 f 70 cm-I (HCBr), and 637 f 80 cm-I (HCI) in the HCX(’A’’) + e- HCX-(R 2A”) transitions are attributed to the C-X stretch of the neutral. Adiabatic electron affinities for the singlet states are found to be 0.542 f 0.005 (HCF), 0.535 f 0.005 (DCF), 1.210 f 0.005 (HCCl), 1.454 f 0.005 (HCBr), and 1.680 f 0.005 eV (HCI). The electron affinity of ’HCI is expected to lie between 1.25 and 1.59 eV. Asymmetry parameters are also reported for photoelectrons from F, Br-, and I- (hv = 351.1 nm). +
+
I. Introduction Carbenes have intrigued chemists for a number of decades, with their fascinating chemistry,’” two low-lying electronic states, and their challenge to ab initio theory. Interesting problems include the determination of carbene geometries, ground-state multiplicities, and the energy difference between the low-lying lA’ and 3Atr states. The chemistry is quite different for these two states. For example, singlet methylene undergoes stereospecific cis addition to olefins while triplet-state methylene undergoes nonstereapecifk addition to olefins. In the early studies of carbene chemistry the degree of stereospecificityin reactions with olefins was used to infer the ground spin state of the carbene.Zc6 Later these types of reactions were used to determine that HCF,’ HCC1,8 and HCBr9 possessed ‘A’ ground states. The halocarbenes (HCX) have been the subject of many ab initio calculations, both on the k ‘A’ state geometries for HCF,’&I8 HCCl,Ie17 and HCBr1e16 and their H 3A’r state geometries.’*15 Computations by Bauschlicher et al.14 including configuration interaction obtained geometries and vibrational frequencies for the ground-state singlet and low-lying triplet spin states of HCF, HCCl, and HCBr. Scuseria et a1.I6 determined molecular geometries and frequencies for the lowest singlet and triplet states of HCF, HCC1, and HCBr using triplet plus double polarization basis sets. Geometries, frequencies and force fields for the k IA’ and the H 3ANstates of HCF were computed by Weis et al.IOusing highly correlated electron wave functions. Tomonari et- al.” obtained geometries, frequencies, and force fields for the X ‘A’ and the g ’A” neutral states of HCF and for the k zA’t state of H C F . Predictions for the HCF tri let excitation energy have varied from 0 to 26.7 k c a l / m ~ l . ~ * ~25~ JRecent calculationsby Weis et al.Io and Shin et alSzpredict the energy difference between the lowest singlet and triplet states to be 13.9 and 14.5 kcal/mol,
E
To whom correspondence should be addressed. Present address: Department of Chemistry, University of Nevada, Reno, NV 89557-0020.
respectively. Additional computations on HCC11e16*23*24,26 and HCBr14J6predict that the triplet excitation energy decreases in (1) Gaspar, P. P.; Hammond, G. S . In Carbene Chemistry; Kirmse, W., Ed.; Academic: New York, 1964; Chapter 12. (2) Gasper, P. P.; Hammond, G.S. In Carbenes; Moss, P. A,, Jones, Jr. M., Eds.;Wiley-Interscience: New York, 1975; Chapter 6. (3) Gilchrist, T. L.; Rees, C. W. Carbenes,Nitrenes, and Arynes; Appleton-Century-Crofts: New York, 1969; Chapter 6. (4) Skell, P. S.;Klebe, J. J . Am. Chem. Soc. 1960, 82, 247. (5) Skell, P. S.; Woodworth, R. C. J . Am. Chem. Soc. 1956, 78, 4496. Skell, P. S.; Woodworth, R. C. J . Am. Chem. Soc. 1956, 78,6427. Woodworth, R. C.; Skell, P. s. J . Am. Chem. Soc. 1959,81, 3381. (6) Closs, G.L. In Topics in Stereochemistry; Eliel, E.L., Allinger. N. L., Us.; Wiley: New York, 1968; p 193. (7) Tang, Y.; Rowland, F. S. J. Am. Chem. Soc. 1%7,89,6420. (8) Hine, J. Diualenf Carbon; Ronald Press: New York, 1964; p 75. (9) Closs, G. L.; Coyle, J. J. J . Am. Chem. Soc. 1965, 87, 4270. (10) Weis, B.; Rosmus, P.; Yamashita. K.; Morokuma, K. J. Chem. Phys. 1990, 92,6635. (1 1) Tomonari, M.; AlmlBf, J.; Taylor, P., private communication. (12) Baird, N. C.; Taylor, K. F. J. Am. Chem. Soc. 1978, 100, 1333. (13) Staemmler, V. Theor. Chim. Acta 1974, 35, 309. (14) Bauschlicher, Jr., C. W.; Schaefer 111, H. F.; Bagus, P. S. J . Am. Chem. Soc. 1977, 99, 7106. (15) Hoffmann, R.;Zeiss, G.D.; Van Dine, G. W. J . Am. Chem. Soc. 1968, 90, 1485. (16) Scuscria. G. E.; DurBn, M.; Maclagan, R. G.A. R.; Schaefer 111, H. E.J. Am. Chem. SOC.1986, 108, 3248. (17) Mueller, P. H.; Rondan, N. G.; Houk, K. N.; Harrison, J. F.; Hooper, D.; Willen, B. H.; Liebman, J. F. J . Am. Chem. Soc. 1981, 103, 5049. (18) Carter, E. A.; Goddard 111, W. A. J . Chem. Phys. 1988,88, 1752. (19) Goldfield, D.; Simons, J. J. Phys. Chem. 1981, 85, 659. (20) Luke, B. T.; Pople, J. A.; Krogh-Jespersen, M.-B.; Apeloig, Y.; Karnie, M.; Chandrasekhar, J.; Schleyer, P. v. R. J . Am. Chem. Soc. 1986, 108, 270. (21) Dixon, D. A. J . Phys. Chem. 1986, 90,54. (22) Harrison, J. F. J . Am. Chem. SOC.1971, 93, 4112. (23) Carter, E. A.; Goddard 111, W. A. J . Phys. Chem. 1987, 91,4651. (24) Carter, E. A.; Goddard 111, W. A. J . Phys. Chem. 1986, 90,998. (25) Shin, S. K.; Goddard 111, W. A,; Beauchamp, J. L. J . Chem. Phys. 1990, 93,4986.
0022-3654/92/2096-1130$03.00/00 1992 American Chemical Society
Spectroscopy of H C F , HCCl-, HCBr-, and HCIthe order HCF > HCCl > HCBr. To our knowledge, no ab initio results on HCI have been published. Additional information has been obtained about the geometries and electronic structures of the halocarbenes through optical spectroscopy. Thtse-studies have focused on the transition between the R ‘A’ and the A lA” electronic states of HCF>7-39DCF,@ and HCC1.28~4143 Merer and Travis’) observed absorption bands of HCF between 4300 and 6000 A in the flash photolysis of HCFBr2. They assigned a 1403.2-m-’ feature to the ground-state bending mode, and determined that HCF is nonlinear in both the upper and lower states, with bond angles of -127O and -102O, respectively. They did not observe any bands corresponding to excitation of the stretching modes. H a k ~ t a obtained )~ a value of 1406.87 (27) cm-’ for u2 (bending mode) by laser-induced fluorescence, in accord with the earlier observation of Merer and Travis.’) In infrared matrix isolation studies of HCF, Jacox and Milligan“ observed absorptions at 1405 and 1181.5 cm-’, corres nding to u2 (bend) and v3 (C-F stretch), respectively, for the ‘A’ state, produced by vacuum-ultraviolet photolysis of CH3F, CD3F, and W H 3 F in argon and nitrogen matrices. Since they were unable to observe any absorption due to the C-H stretching mode, ul, they calculated force constants for the 2 ‘A’ state using assumed values for v l . Recently Suzuki and H i r ~ t a observed )~ the C-H stretching mode of HCF (2643.0393 (26) cm-’) in a stimulated emission pumping experiment and improved the force field of Jacox and Milligan“ for HCF. Merer and Travis42analyzed the band system between 5500 and 8200 A arising from the photolysis of HCC1Br2. They attributed this band system to the A ‘A” % ‘A’ transition of HCCl and determined the ground-state geometry of HCCl. Kakimoto et al.41obtained a rotational spectrum of HCCI. They carried out a normal-coordinate analysis and calculated centrifugal distortion constantsand the inertial defect. Their HCCl ground-state force field is in reasonable agreement with that obtained by Jacox and Milligan.4s Results are not available from optical spectroscopy on HCBr and HCI. A previous photoelectron spectroscopy study on the halocarbene and CC12and CF2anions from this laborat09 yielded vibrational frequencies for the C-X stretch of the anions and singlet neutrals. Adiabatic electron affinities were found for HCF, DCF, and HCC1. Tentative origin assignments were also made for HCBr and HCI. Upper bounds of 14.7 f 0.2 kcal/mol (HCF, DCF), 11.4 f 0.3 kcal/mol (HCCl), and 9 f 2 kcal/mol (HCBr) were given for the triplet excitation energies. More precise values could not be obtained, because the portion of the spectrum corresponding to the triplet origin was obscured by much stronger transitions to excited singlet vibrational levels. The upper limits were based on the values that produced significant deviations of the Franck-Condon singlet simulation from the data. Only an ab-
Ip”
-
(26) Shin, S.K.; Goddard 111, W. A,; Beauchamp, J. L. J . Phys. Chem. 1990. 94. 6963.
(27) AshGld, M. N. R.; CastaAo, F.; Hancock, G.; Ketley, G. W. Chem. Phys. Len. 1980, 73, 421. (28) Qiu, Y.; Zhou, S.; Shi, J. Chem. Phys. Lerr. 1987, 136, 93. (29) Dixon, R. N.; Wright, N. G . Chem. Phys. Lerr. 1983, 100, 311. (30) Patel. R. I.: Stewart. G.W.: Casleton.. K.:. Gole.. J. L.:. Lombardi. J. R. Chem. Phys. 1980, 52,461. (31) Suzuki,T.; Saito, S.; Hirota, E. Can. J. Phys. 1984, 62, 1328. (32) Kakimoto, M.; Saito, S.;Hirota, E. J. Mol. Spcrrosc. 1981,88, 300. (33) Merer, A. J.; Travis, D. N. Can. J. Phys. 1966, 44, 1541. (34) Hakuta, K. J . Mol. Specrrosc. 1984, 106, 56. (35) Dixon, R. N.; Wright, N . G. Chem. Phys. Lerr. 1983, 100, 311. (36) Butcher, R. J.; Saito, S.; Hirota, E. J . Chem. Phys. 1984,80, 4000. (37) Suzuki,T.; Hirota, E. J . Chem. Phys. 1986, 85, 5541. (38) Ibuki, T.; Hiraya, A.; Shobatake, K.; Matsumi, Y.; Kawasaki, M. J. Chem. Phys. 1990, 92,4277. (39) Suzuki,T.; Hirota, E. J . Chem. Phys. 1988, 88, 6778. (40) Suzuki, T.; Saito, S.;Hirota, E. J. Mol. Specrrosc. 1981, 90, 447. (41) Kakimoto, M.; Saito, S.; Hirota, E. J. Mol. Specrrosc. 1983,97, 194. (42) Merer, A. J.; Travis, D. N . Can. J . Phys. 1966, 44, 525. (43) Hirota, E. Faraday Discuss. Chem. Soc. 1981, 71, 87. (44) Jacox, M. E.; Milligan, D. E. J. Chem. Phys. 1969, 50, 3252. (45) Jacox, M. E.; Milligan, D. E. J . Chem. Phys. 1967, 47, 1626. (46) Murray, K. K.; Lcopold, D. G.;Miller, T. M.; Lineberger, W. C. J . Chem. Phys. 1988.89, 5442. ’
The Journal of Physical Chemistry, Vol. 96, No. 3, 1992 1131 solute value of the upper bound for the first excited state of HCI (9 f 2 kd/mol) was given, since the ground-state spin multiplicity of HCI was not known. In this study we use the same flowing afterglow negative ion photoelectron spectrometefl6 to examine the HCX(’A’) HCX-(f( 2A“) and HCX()A”) HCX-(X 2A”) transitions (X = F, C1, Br, and I) in more detail. There are two major differences between this study and the previous one. First, we use 351.1 nm (hv = 3.531 eV) radiation rather than 488-nm (hv = 2.540 eV) laser light, which allows us to study higher vibrational levels of the triplet states. Second, with the new laser system we can rotate the laser polarization using a X/2 plate, permitting us to study the angular distributionsof the photoelectrons. Polarization studies aid in identifying the symmetries of transitions in the photoelectron spectrum of HCI and can provide a means to separate the overlapping singlet and triplet states. The outline of the paper is as follows: In section I1 we describe the experimental techniques, the current laser system, and the method of ion production, and include a brief explanation of the angular distributions of photoelectrons. In section I11 we discuss our experimental results which include spectra taken at several laser polarizations and values for the asymmetry parameters. Rotationally corrected adiabatic electron affinities are presented in section IV. A discussion of the Franck-Condon analyses and the determination of anion geometries is also in this section, followed by a detailed analysis of the individual halocarbenes and a comparison of our results with previously reported experimental and theoretical results. We also include a qualitative discussion on triplet excitation energies and on the angular distributions of photoelectrons for the singlet and triplet states of the halocarbenes. We summarize our results in section V.
-
-
IT. Experimental Section
.-
A Wotoelectron The photoelectron spectrometer has been described in detail re~ently;~’.~~ therefore, a short explanation suffices here. Briefly, negative ions are produced in a flowing afterglow s0urce,4~accelerated, focused into an ion beam, mass selected with a Wien velocity filter, and crossed by a continuous laser beam of fixed photon energy. Photodetached electrons are collected perpendicularly to the ion and laser beams. The kinetic energies of the photodetached electrons are measured with a hemispherical electrostatic energy analyzer with a resolution of 9 meV. The spectra are obtained by measuring the kinetic energy of photodetached electrons from the process
HCX-(u’?
+ hv
-
HCX(u9
+ e-
where u“ and u‘ denote the vibrational states of the ionic and neutral species. The spectra are taken as a function of electron kinetic energy ( e m ) , and are converted to binding energy (eBE), which is the difference between the photon energy and the electron kinetic energy (eBE = hv - e m ) . Therefore peaks at low binding energies correspond to high-energy photoelectrons. The absolute energy scale of the spectrometer is calibrated with 0-(EA = 1.461 121 5 (10) eV).So In addition a small linear correction is determined,s1which accounts for the relative energy scale compression of the hemispherical electron energy analyzer (typically about 0.5%) by photodetaching W- and comparing the W fine structure splittings observed in the W- photoelectron spectrum to known values.s2 The experimental uncertainty is f0.005 eV for the absolute electron kinetic energy of well-resolved peaks in the photoelectron spectra. (47) L,eopold, D. G.; Murray, K. K.; Stevens Miller, A. E.; Lineberger, W. C. J. Chem. Phys. 1985,83, 4849. (48) Ervin, K. M.; Ho, J.; Lineberger, W. C. J. Chem. Phys. 1989, 91, 5974. (49) Ferguson, E. E.; Fehsenfeld, F. C.; Schmeltekopf,A. L. Ado. AI. Mol. Phys. 1969, 5, 1 . (50) Neumark, D. M.; Lykke, K. R.; Andersen, T.; Lineberger, W. C. Phys. Rev. A 1985, 32, 1890. (51) Feigerle, C. S.Ph.D. Thesis, University of Colorado, 1983. (52) Moore, C. E. Afomic Energy Leuels; Natl. Stand. Ref. Data Ser. Natl. Bur. Stand. No. 467; US.GPO Washington, D.C., 1958.
1132 The Journal of Physical Chemistry, Vol. 96, No. 3, 1992 9. Ultraviolet Laser System. The 351-nm output of a single-mode argon-ion laser is amplified in an optical buildup cavity, where the mirrors of the cavity also form the windows of the chamber!* The buildup cavity is a high-finesse Fabry-Perot interferometers3that is electronically stabilized and locked to resonance. A servoamplifier system matches the resonant frequencies of the laser and the buildup cavity by adjusting the lengths of the two cavities (laser and buildup cavities) with piezoelectric translators on the cavity mirrors. When starting with 150-200 mW (single frequency, single mode) of incident laser power, we routinely achieve buildup factors of 150-250, producing 30-50 W inside the buildup cavity. Rotation of a X/2 plate external to the buildup cavity rotates the electric vector of the laser radiation relative to the direction in which the photoelectrons are detected. C. Angular Distribution of Photoelectrons. The differential photodetachment cross section using linearly polarized light in the electric dipole approximation is generally written ass4
du/dSl = (u0/4?r)[l
+ j3P2(cose)]
(1)
where uo is the total photodetachment cross section, P2(cos e) = (3 ax20 - 1)/2, e is the angle between the direction of the ejected electron and the polarization of the incident light, and @ is the asymmetry parameter (-1 I @ I +2). At the *magic" angle of e = 54.7*, P2(cos 0) is equal to zero and the photodetachment signal is proportional to the total photodetachment cross section. We have taken full photoelectron spectra for H C F , HCCl-, HCBr-, and HCI- at three different laser polarizations, 8 = 90°, 8 = 54.7O, and e = Oo, giving approximate asymmetry parameters. More precise values were obtained for the asymmetry parameter, 8, at a fixed photoelectron energy by changing e in 5-10° increments through several periods and monitoring the photoelectron intensity normalized by laser power and ion current. A leastsquares fit of the data to eq 1 was used to obtain the best value for 8. D. Ion Roductioa Halocarbene anions were produced in the flowing afterglow source by reacting 0-with a singly halogenated methane (CH3X). The 0-ions (300-1000 PA) were produced in a 2.45-GHz microwave dmharge of helium seeded with either 025or N20?9 Typical flow rates were between (3-10) X IO3 std cm3min-' of the helium buffer gas and 5 std cm3min-' (N20) or 10 std cm3min-' (02).After optimizing the 0-ion signal, 1-5 std cm3 min-' of the singly halogenated methane, CH3X, was introduced downstream of the microwave discharge. The 0reacted with the halogenated methane by H2+abstraction:
0-+ CH3X
-.
H20
+ HCX-
to produce the halocarbene anion.s658 Ion currents following
mass selection ranged between 60 and 165 PA for the haloearbenes. By varying the inlet position for the singly halogenated methane, inmasing or decreasing the helium buffer gas pressure, and adding 100-400 std cm3min-' of Ar, we were able to either cool or heat the anions vibrationally. Spectra taken at different vibrational temperatures were used as an aid in the confirmation of origin assignments. In addition to 0-, a considerable quantity of the 0,ion was also produced in the flowing afterglow source when O2was used to generate 0-.Since HCF and 02-have the same mass and the interpretationof minor overlapping photoelectron spe~tra,~5~@
Gilles et al.
Electron Kinetic Energy (eV) 1 .o
2 .o
HCF
2111
-
0.1
3 0
0.1
2
U
p
-
0.4
I
t
2
0.i
a
0 .(
;111; 0.8
-.
HCCl'
1
ucr-
L
a 0 V
e 0 .f
2 p
U
O.r
W
0
CI
2
0.2
a
0 .c 111 CI
# 0 .E
a 0 V
# 0 .E
0 CI
u W
0.4
I
t
U
2 a
0.2
0 .o 111 g
0.8
3 0
V
0.6
2
-p U
0.4
W
0 c)
2
0.2
& 0 .o
3 :O
1:o
2:o
Electron Binding Energy (eV) Figure 1. Photoelcctron spectra of the halocarbenes, H C F , HCCI-, HCBr-, and HCI- taken at the "magic angle" with hv = 3.531 eV. Photoelectron counts are plotted as a function of the electron binding energy, the difference between the photon energy and the measured electron kinetic energy. The HCX('A') eHCX-(k 2At') transitions are seen at lower electron binding energies and the vibrational origins are indicated by an arrow. The vertical HCX()At') + eHCX-(% 2A'') transitions are seen to the left, at higher electron binding energies.
+
(53) Siegman, A. E. In Losers;University Science Book. Mill Valley, CA, 1986; pp 413ff. (54) Cooper,J.; Zare, R. N. J . Chem. Phys. 1%8,48,942; erratum, 1968, 49, 4252. (55) Bohme, D. K.; Fehsenfeld, F. C. Cun. J. Chem. 1%9, 47, 2717. (56) Tanaka, K.; Mackay, G . I.; Payzant, J. D.; Bohme, D. K. Cun. J. Chem. 1976,54, 1643. (57) Dawson, J. H. H.; Jennings, K. R. J . Chem. Soc., Furuduy Trum. 2 1976. - - . - , .72. 7. -M-.. (58) Grabowski, J. J.; Melly, S. J. Inr. J. Muss Spectrom. Ion Proc. 1987, 81, 147. (59) Celotta, R. J.; Bennett, R. A.; Hall, J. L.; Siegel, M. W.; Levine, J. Phys. Rev. A 1972, 6, 631.
3 .O
1 .I
-
-
features in the H C F spectrum can easily be compromised by an 02-contaminant. Several steps were taken to minimize the contaminant 02-present in the ion beam. First, because N20 (60) Travers, M. J.; Cowles, D. C.; Ellison, G. B. Chem. Phys. Lett. 1989, 164, 449.
The Journal of Physical Chemistry, Vol. 96, No. 3, I992 1133
Spectroscopy of H C F , HCCl-, HCBr-, and HCIproduces less O,; NzO was used as the source of 0-.Second, after taking a spectrum of H C F (with contaminant O r ) , the CH3Fwas shut off and a spectrum of only 0; was taken. This spectrum was normalized to the peak at 0.644eBE and subtracted from the contaminated H C F spectrum. For all of the HCF spectra we discuss below the contaminant Of spectrum has been subtracted. Typically the 0;contamination was less than 20%. The halogen atomic anions were normally present in the ion beam when the halogenated compounds were used in the ion source. The F,Br, and I- currents present in the ion beam,when the singly halogenated methanes were used to make the halocarbenes, were sufficient for measurement of their asymmetry parameters.
TABLE I: Asymmetry Parameters for Photodetachment of HCXand X- (X = F, CL Br, and I) neutral species ‘A‘
anion species HCF HCCIHCBrHCI-
peak”
B
2.830 2.015 1.899 1.776
0.14 f 0.1 -0.30 f 0.1 -0.50 f 0.1 -0.67 f 0.1
A. Spectra Taken at the Magic Angle. The 351.1-nm photoelectron spectra of the halocarbenes displayed in Figure 1 were obtained with the laser polarization at the ymagic angle”. Transitions from the anion ground state to two neutral electronic states are seen in all of the spectra in F w e 1. The photoelectron spectra of H C F , HCCl-, and HCBr- exhibit a vibrational progression in the C-X stretchin mode (v3) of the neutral, arising from the transition from the zA” ground state of the anion to the ground-state neutral, % ‘A’. The second vibrational progression, at higher binding energies, arises from the photodetachment to the first excited state of the neutral, H 3A”. The transition to the H 3A” electronic state appears to consist of a single active vibrational mode for HCF; however, it will be seen from the isotopically substituted DCF that there are two nearly degenerate modes active in this progression. The photoelectron spectra of the H 3A” state for HCCl and HCBr display congested vibrational progressions arising from the two active vibrational modes. The HCI- photoelectron spectrum exhibits structure similar to that of HCCl- and HCBr-, that is the appearance of a single vibrational progression in the state at lower bmdw energies while the state at higher binding energies is severely congested. This spectrum additionally exhibits a sharp peak at 3.059 eV, the binding enerd’ of I-. This peak arises from a two photon process, consisting of the photodissociation of HCI- into HC and I-, followed by the subsequent photodetachment of I- to I e-. This results in the appearance of a photoelectron at the correct energy for detachment of free I-. Similar twephoton processes have been observed previously for 03-and Au
