J. Phys. Chem. 1995,99, 1637-1642
-
Threshold Photoelectron Spectroscopy of CO+(A 211j)
1637
CO(X lZ+)
W. Kong and J. W. Hepburn" Centre for Molecular Beams and Laser Chemistry, Department of Chemistry, University of Waterloo, Waterloo, Ontario N2L 3G1, Canada Received: August 30, 1994; In Final Form: October 21, I994@
The threshold photoelectron spectra for the lowest vibrational levels of the CO+ A 211state are presented and discussed. These spectra were recorded with single-photon excitation of jet-cooled CO molecules, using pulsed field ionization to detect the ionization thresholds with rotational resolution. One of the sub-bands, corresponding to the A2111,2(v+ = 0) level, is perturbed by the X 2 F (v+ = 10) level, and the spectrum appears quite different from the corresponding v+ = 1 band. An explanation based on core relaxation induced by Rydberg electron-ion core collisions is invoked to explain the observed spectrum. For the other unperturbed bands, a simulation based on a standard model for rotational line strengths in photoelectron spectra provides a satisfactory description of the spectra. The results of this simple model are compared with detailed ab initio calculations.
1. Introduction
Pulsed field ionization threshold photoelectron spectroscopy, of PFI-ZEKE (for zero kinetic energy) spectroscopy, has revolutionized photoelectron spectroscopy, enabling rotational structure to be resolved in the photoelectron spectra of a wide range of small molecules.' In the past few years, both theoretical and experimental studies have been quite extensive, focusing for the most part on the spectroscopy and ionization dynamics studies of the ground electronic state of small molecular ions?-5 However, not many studies of electronically excited ionic states have been r e p ~ r t e d . ~In, ~a previous paper,' we presented our first study of electronically excited ionic states using PFI-ZEKE. This paper on the NO+(a 3Z+) state showed, among other things, the effect of a complex resonance on the relative intensities of vibrational bands in the PFI-ZEKE spectrum, and the role of vibrational autoionization in the rotational line strength was addressed. In this paper, we report results on the ionization dynamics of CO+(A 211jV + = 0, 1) using single-photon ionization with a tunable coherent vacuum ultraviolet light source. This work complements our previously published experimental and theoretical study on the ground state of CO+.5 The paper following this will describe detailed ab initio calculations based on the current experimental results.* In this paper, we shall use an empirical theory to describe our measured intensities. The spectroscopic aspects of carbon monoxide and its molecular ion are well-known because of their considerable importance in astrophysics?%1oIonization dynamics of CO have also been investigated using discharge lamps,lI synchrotron radiation,I2 and REMPI technique^.'^ However, detailed autoionization mechanisms of the Rydberg states converging to the A 211 state are still not fully understood. The ionization threshold of CO+(A 21T v f = 0 ) is 16.5 eV, corresponding to ionization of the ln electron from the ground state of CO. The final ionic state belongs to Hund's coupling case a, with a spin-orbit splitting of -120 cm-'. Without spin-orbit autoionization,the relative intensities of the two components are expected to be nearly the same.I4 We shall use the direct ionization model originally developed by Buckingham, Om, and Sichel (the BOS model)I5 to model @
Abstract published in Advance ACS Abstracts, January 15, 1995.
0022-365419512099-1637$09.0010
our measured spectra, to maintain consistency with our own and other published PFI-ZEKE work. Although this model should apply to excited states of ions, there has been little work done using the BOS model to simulate PFI-ZEKE spectra of excited state thresholds. Also of interest to this work is the interpretation of the results of using the BOS model in an empirical fashion, using the matrix elements an adjustable parameters to fit spectral intensities. With the current results, we will be able to compare the BOS results with accurate ab initio work on the same system. High-resolution spectroscopic studies have shown that the A 2111/2 v+ = 0 level is perturbed at low J by the X 2Z+ v+ = 10 level.I6 This perturbation mixes the wave functions of the two states, resulting in a change in the collisional relaxation rate and fluorescence lifetime for perturbed rotational levels of the A 2 1 1 ~ / 2v+ = 0 state. In PFI-EKE studies, this perturbation involves the ionic core, and perhaps the Rydberg electrons as well. This interaction is different from complex resonance in the sense that the two interacting Rydberg series are both with high principal quantum numbers. This perturbation has a dramatic effect on line positions and line intensities in the PFIZEKE spectrum. In this paper, single-photon ionization studies of CO+(A 211 v+ = 0, 1) CO(X lZ+)will be presented. First, the PIE spectrum and the rotationally resolved PFI-ZEKE spectra will be shown, illustrating the complexity of the system. Assignment of the perturbed A 2111/2 v+ = 0 band using the Hamiltonian of Coxon et al.I7 and the deperturbation calculation will also be presented. In order to understand the observed rotational structure, detailed ionization dynamics will be examined. For the unperturbed bands, rotational intensity distributions will be simulated using the BOS model, and comparisons between the different spin-orbit components and final vibrational levels will be addressed. In the following paper, a detailed theoretical study on this system is presented.
-
2. Experimental Results and Spectroscopic Analysis The apparatus used in these experiments has been described in detail in previous publication^.^.^^^^ In brief, the experiments were done using broadly tunable coherent vacuum ultraviolet radiation, generated by four-wave sum-mixing in pulsed jets, to excite molecules in a collimated supersonic molecular beam. 0 1995 American Chemical Society
Kong and Hepbum
1638 J. Phys. Chem., Vol. 99, No. 6, 1995 '
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133000
134000 VUV Energy ( c m - ' )
135000
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Figure 1. PIE spectrum of jet-cooled CO in the region of the A 211v+ = 0 and 1 ionization thresholds. Rydberg states converging to various vibrational states (v') of the A 217 state are labeled, as are the ionization limits determined in the current work (arrows).
Coherent vacuum ultraviolet radiation in the energy region 16.48-16.75 eV was generated in a pulsed jet of Kr. The 5p[1/2,0] two-photon resonant level of Kr at 94093.7 cm-I (201) was used for resonant enhancement,and 02 was generated using another dye laser with a BBO @-barium borate) doubling crystal, tunable between 40 984 and 38 760 cm-'. The generated photon frequency was wvuv = 201 -I- 0 2 . The similarity between 0 1 and 0 2 made it impossible to overlap the two fundamentals using a dichroic mirror, thus the optical path was arranged as follows: The fundamental of 01(01/2) directly from the dye laser was transmitted by the dichroic mirror and overlapped with 0 2 , which was reflected by the dichroic mirror. A BBO doubling crystal was set after the dichroic mirror to generate 01. Although the 02 beam also passed through the BBO crystal, it was not affected by this crystal. Photoions and photoelectrons were detected by a double time of flight spectrometer. The delayed pulsed electric field (0.8 V/cm) was applied 1 ps after the vacuum W pulse for the PFIZEKE spectra. The field-ionized electron signals were recorded by a gated integrator. When recording the photoions for the photoionization efficiency (PIE) spectrum, a DC extraction field was set at 16 V/cm on the ion spectrometer. In both types of spectra, the vacuum ultraviolet intensity was monitored, and the spectra were all normalized by relative vacuum ultraviolet intensity. Figure 1 shows the PIE spectrum near the thresholds of v+ = 0 and 1. Assignment of the Rydberg states is based on the work of Ogawa and Ogawa,I9 and the thresholds are marked according to the results of the present PFI-ZEKE work. The PIE spectrum is similar to that obtained by Hardis et al. using a continuous helium lamp." The main differences are in the resolving power and rotational temperature. The optical resolution of Hardis et al.'s work was 0.07 8,or 12 cm-', the rotational temperature was 78 K, and thus in their spectrum some of the Rydberg states were not fully resolved. The increased resolution of the coherent vacuum UV source and the reduced rotational temperature results in most of the Rydberg states being clearly resolved, with some showing fine structure. Although there are weak autoionizing resonances near all the ionization thresholds investigated in the current study, the resonance which overlaps the 21T1,2 vt = 0 threshold is by far the strongest of these. We would thus expect the rotational structure of the PFI-ZEKE spectrum for the 2113/2 v+ = 0, 2111/2, and 2113,2 v+ = 1 bands to be much less perturbed by nearby
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VUV Energy ( c m - ' ) Figure 2. PFI-ZEKE spectrum of the A2173,2 (v+ = 0) state and corresponding BOS simulation. Branches are labeled: AJ(J"), as described in the text. 0.30
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VUV Energy ( c m - ' ) Figure 3. PFI-ZEKE spectrum of the A *II3,2 (v+ = 1) state and corresponding BOS simulation. Intensity scale the same as Figure 2.
autoionization resonances than the 2111/2 v+ = 0 bands7 The top halves of Figures 2-4 show the experimental data for the three sub-bands, and the bottom halves are the simulation results
Photoelectron Spectroscopy of CO+(A 211J
-
CO(X ‘Z+)
J. Phys. Chem., Vol. 99, No. 6,I995 1639 -PFI-TPES
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Figure 5. PFI-ZEKE spectrum of the perturbed A 2 K I ~ / 2(v+ = 0) state (solid line) and PIE spectrum (dashed line). Line positions for transitions to “X 2Z+”levels are shown by the dashed lines, those to “A 21T1/i’ levels are indicated by solid lines. Intensity scale for the PFI-ZEKE spectrum is the same as Figure 2. TABLE 1: Deperturbation Results energy level J+ (cm-’)
135000
135040
VUV Energy (cm-’) Figure 4. PFI-ZEKE spectrum of the A 2 1 7 ~ / 2(v+ = 1) state and corresponding BOS simulation. Intensity scale the same as Figure 2. using the BOS m0de1.I~ The vertical axes indicate the relative intensities of these sub-bands. The labeling refers to the difference in the total angular momentum of the ionic state J+ and the ground state S’, AJ = J+-S’. For convenience, a transition with a total angular momentum change of A J from the .I”level of the ground state will be referred to as AJ(s’). Assignment of the rotational structure in the PFI-ZEKE spectra was based on known spectroscopic constants.20 For the 2113/2 (v+ = 0, 1) and 2111/2 (v+ = 1) bands, this assignment was straightforward and unambiguous. The observed maximum change in rotational angular momentum is 7/2. For the simulations shown, the relative positions of the various rotational lines were calculated from the published spectral constants, and the band origins were adjusted to fit the observed thresholds. The relative line intensities were fit using the BOS model,I5 assuming a rotational temperature of 10 K, as discussed in the next section. The PFI-ZEKE spectrum of 2111/2 v+ = 0 is shown in Figure 5, together with the PIE spectrum (dashed line). It is obvious that the rotational structure of this band is quite perturbed, both in terms of line positions and line intensities. Perturbations in the low rotational energy levels of 2111/2 v+ = 0 have been observed and documented by Katayama et a l . I 6 Using the perturbation Hamiltonian2I and the deperturbation results of Coxon et al.,” the calculated energy levels and the mixing percentages in the wave functions of the two sets of rotational levels are listed in Table 1. The perturbed states are in quotes, and the term symbols indicate the major component in the wave function. Mixing with the e levels of the 2113/2 component is negligible and not listed here. Between the two A doublet components, only the e levels participate in the perturbation. The f components of the “A state” rotational levels are not perturbed but are all within 0.4 cm-’ of the e components, due to the weakness of the perturbation. The term value difference between the two perturbing states, T, was used instead of the
133 495.6 133 498.1 133 500.4 133 501.7 133 508.2 133 508.9 133 519.9 133 519.2 133 534.2 133 533.4 133 551.8 133 551.1 133 572.2 133 572.9
character in the wave function (%)
2n1/2
98.2 1.7 93.3 6.6 66.6 33.4 71.9 28.0 80.6 19.4 72.1 27.4 65.8 34.0
absolute values, and the obtained energy levels were converted into the ionization thresholds observed in the PFI-ZEKE spectrum. The shift caused by the electric field was also included in the listed energy levels, to allow for a direct comparison with the measured PFI-ZEKE spectrum. From Table 1, the rotational line positions were calculated for the “2J3~/2)’v+ = 0 band and the “2Z+”vf = 10 band, and the results are shown in Figure 5. The solid vertical lines in Figure 5 correspond to transitions to the “A state” and the dotted lines to the “X state”. It should be pointed out that although the absolute uncertainty of the vacuum W photon energy is 1.5 cm-I, the uncertainty in the relative line positions measured from a single spectrum is lower, less than 0.5 cm-I. The line width of the central strong peak in the spectrum is bigger than the others, and several lines in the 1/2 branch must have contributed to the signal strength. However, while the presence of a strong 1/2 branch is a feature in the unperturbed bands, the position of this branch in Figure 5 is in alignment with the “2Z+” state lines and not the “2111/2)’ lines. More detailed spectral simulations show that the major portion of this “133 497.9 cm-’” peak is due to low J” lines of the “2Z+” 1/2 branch, with contributions from the cc21T~/2”1/2 branch 1.3 cm-I to the red and the “ 2 1 T l / ~ ’3/2(0) line 2.5 cm-’ to the blue. To the red of the major peak is a peak at 133 492.1 cm-’, which must be assigned to the “21T1/2” -1/2(1) line, and an unresolved cluster of peaks belonging to the -1/2 and -312 branches. The 3/2(1), 5/2(1), 5/2(2), and 7/2(0) lines are best assigned as
,
,
,
1640 J. Phys. Chem., Vol. 99, No. 6, 1995
Kong and Hepbum
TABLE 2: Line Positions in the PFI-TPE Spectrum transition
experimental
x 2z+
- 1/2( 1) 1/2(0) 1/2(1) 1/2(2) 1/2(3) 3/2(1) 5/2(0) 3/2(2) 5/2( 1) 5/2(2) 7/2(0) 5/2(3)
133 492.2 133 497.9 133 497.9 133 497.9 133 497.9 133 505.2 133 508.6 133 508.6 133 515.3 133 522.0 133 519.2 133 528.7
133 494.3 133 498.2 133 497.9 133 497.5 133 496.2 133 505.2 133 509.0 133 507.7 133 515.4 133 522.0 133 519.2 133 528.2
A 2n1/2 133 491.9 133 495.7 133 496.6 133 496.8 133 496.9 133 504.5 133 508.3 133 508.4 133 516.1 133 522.8 133 520.0 133 528.9
transitions to the “X’ state. The 5/2(0) line and the 3/2(2) line are similar in position, and the observed transition could be to the X state J+ = 7/2 and 9 2 , or, though less likely, to the A state. The weak 5/2(3) line can be assigned to both states within the uncertainty. The assignments are summarized in Table 2, which uses the energies from Table 1 to calculate line positions for the transitions to the perturbed rotational levels. 3. Discussion 3.1. PFI-ZEKE of 211j v+ = 1 and 2113/2 v+ = 0. The rotational distribution of the unperturbed sub-bands was fitted using the BOS m0de1.l~ A detailed description of the fitting procedure was discussd in ref 7. Basically, the rotational line strength was separated into two factors. One is only related to the radial wave functions of the initial and final states, and the other is determined by angular momentum coupling algebra. The fitting procedure adjusts the radial factor to match the experimental rotational distribution. The angular momentum coupling factor Q(A) for the Hund’s case a b transition is expressed as +
Q(A) = (2J+
+ 1)(2S+ +
c
l+1/2
l)-l
(2x
+ 1) x
where the definitions of the symbols follow the convention of Hund’s coupling cases a and b. Using the basis set of single center spherical wave functions, the one-electron wave function of the ionized electron in the neutral ground state can be expanded into a sum of orbitals with different orbital angular momenta 1 = A. Thus, in the BOS formulation the expansion coefficients, CA,represent the contribution of the eigenfunction with an orbital angular momentum of A in the PFI-ZEKE signal. However, we have used these coefficients as adjustable fitting parameters, meaning that the magnitude of Ci, includes not only the expansion coefficient of the ground state electronic wave function and the transition moment of the orbital with angular momentum quantum number A but also incorporates effects of possible interactions after photon absorption. These effects include any interaction of the Rydberg electrons with the ionic core, such as autoionization and predissociation, following excitation. As an example, the nonspherical ionic core will give rise to coupling between different rotational channels, and this effect is ignored in the BOS formulation. Thus, while the BOS simulation provides an accurate account of the frame transformation component of the rotational line strengths, it can only be expected to provide qualitative information about the detailed ionization dynamics. For the transition from a E state to a II state, AA = 1, and the first 3-j symbol determines that A 2 1, indicating that the
TABLE 3: Optimized Coefficients for BOS Spectral Simulations state
v+
CI
c 2
c3
2n3/2 2n3/2 ’n1/2
0 1 1
0.67 f 0.02 0.73 k 0.03 0.62 0.02
0.18 f 0 . 0 4 0.21 rt 0.04 0.28 f 0.04
0.14f0.05 0.06 rt 0.04 0.07 h 0 . 0 5
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contribution from the Co term in the ionization process is strictly zero. The agreement between the calculated and measured spectra in Figures 2-4 are generally satisfactory, with most of the main structures reproduced. Table 3 lists the fitting coefficients for the three sub-bands. Major contributions in all three sub-bands come from the C1 terms, meaning the ground state wave function of the ionized electron has mostly p-orbital character. In comparison with the fitting results of the ground electronic state, the wave function of the 1 n orbital is simpler and does not involve as much orbital hybridization. In the theoretical study done by Wang and McKoy,*the more realistic treatment of the ionization dynamics gave qualitatively similar results to those obtained with the simple BOS model. The high value of the C1 coefficient in the BOS simulations can be interpreted as large p-orbital character in the initial orbital that the photoelectron is ionized from. This p character would mean that d-waves and s-waves would dominate the ionization, which agrees with the ab initio results, which indicate a dominant d-wave contribution to the continua. However, beyond this qualitative agreement, there is a significant difference between the BOS simulations and the ab initio results at the rotational temperature used. We found that 10 K provided the best results in our treatment, whereas Wang and McKoy used 50 K, mostly to increase the relative intensity of the +1/2 branch for the ’H3/2 sub-bands. Our experience would indicate that 50 K is too high for the rotational temperature, but equivalently, 10 K may be too low. Since we did not make a simultaneous determination of the CO rotational temperature while the PFI-ZEKE spectra were being recorded, and since previous work on CO and NO beams done under similar conditions produced temperatures in the range 10-20 K, the exact value of Trotation for the current results is uncertain. Although it is more dramatic in the 10 K simulations, even with an assumed 50 K temperature, the experimental intensities for the negative AJ branches are high, compared with the positive AJ branches. As both the positive and the negative A J branches are related to the same transition dipole moment and the same set of expansion coefficients for the ground state wave function, the fitting quality should be the same for both. The experimental data give consistently stronger negative AJ branches than the fitting results, indicating the existence of “forced rotational autoi~nization”.~,~~ This fact suggests that even for electronically excited states, forced rotational autoionization is also possible. As pointed out in the Introduction, the BOS model does not include effects of the ionic core wave function on the final state rotational distribution, and the two spin-orbit components from the v+ = 1 level should give the same fitting coefficients. In Table 3, the difference in the fitting coefficients is only slightly outside the error limits. The relative intensities of the two components are also expected to be identical according to the BOS model. However, the 2113,2 component is clearly stronger than the 2111/2 component for v+ = 1, and the opposite is true for v+ = 0. These differences cannot be attributed to spinorbit autoionization, since according to previous studies,23this autoionization mechanism gives a consistently stronger ionization yield for the component with lower energy. The fitting coefficients are similar for the different v+ levels, unlike the case of NO+(a3E).’ The maximum value of the
Photoelectron Spectroscopy of CO+(A 211,)
-
CO(X
Franck-Condon integral between the neutral ground state and the A state is at v+ = 2,24but the PFI signal at this vibrational level was barely observable. This is an example that even without complex resonance, the PFI signal strength still does not follow the Franck-Condon factors. 3.2. PFI-ZEKE of 2111,2 v+ = 0. The Franck-Condon integral between the neutral ground state wave function and the v+ = 10 level of the unperturbed 2Z+state is virtually zero,24 and direct transition to the “2C+” state should mainly be a result be a result of the 2171/2 vf = 0 component in the perturbed wave function. Thus, in a rotationally resolved ionization spectrum, two overlapping vibrational bands are generally expected; one is associated with the “2C+’’state and the other with the “2111/2” state. Although many of the lines in these two bands are unresolvable with the resolution of the current PFI-ZEKE spectra, it is still possible to state that not all possible lines are seen, and it is certainly true that there has been a large distortion in the relative intensities of the rotational lines in this sub-band. Although the assignments presented in section 2, and discussed further below, are not absolutely certain, because of the limited resolution of the current data, they have been confirmed by simulation of the measured spectra. In all cases, changes to the assignments, or inclusion of additional lines with comparable intensities, reduced the quality of agreement between the simulated and measured spectra. “Extra lines” belonging to the X “2C+” v+ = 10 state were observed by Katayama et al. when measuring laser-induced fluorescence of the A “211,/2” v+ = 0 state,l6 but the observed rotational structure of the A state was complete with no absence of certain transitions. In an effort to explain the different fluorescence decay profiles of the e and f levels, they suggested that mixing between the e levels of the A state and the X state provided a pathway for energy transfer. Fast decay of the A state ions into the long-lived X state can actually deplete the population of the A state, and the observed single-exponential decay of the e level fluorescence is in fact the fluorescence profile of the “*2+”state. No estimation of the relaxation rate was given by the authors, but since this process does not register in the fluorescence time profile, it has to be much faster than 1 p s . In a later paper by Dentamaro et al., collision-induced transitions between the A “2111/2” v+ = 0 state and the X “22+” v+ = 10 state were studied.25 After exciting the J+ = 13/2 level of the A state, a relatively large population at J+ = 13/2 of the X state was detected 10 ns later. They concluded that in spite of the small Franck-Condon factor between the two states, rapid collisional transfer must have happened. In the case of the transitions to . I + > 5/2 (5/2(1), 5/2(2), 7/2(0), and 5/2(3) lines), the spectra show peaks at the “X’ state thresholds, consistently at lower energy than the corresponding “A”state thresholds. One possible explanation for this behavior is that the high Rydberg states converging to the higher “A’ state limits for J+ = 7/2, 9/2, and 11/2 decay by autoionization into the corresponding “X’ state continua, a relaxation of the “A’ state core caused by collision with the Rydberg electron, analogous to the relaxation observed in the LIF experiments, caused by collisions with the surrounding molecules. According to classical atomic orbital theory, a Rydberg electron with a principal quantum number, n, can collide with the ionic core at a frequency of 6.6 x lOI5/n3H z . ~Thus ~ for Rydberg electrons with principal quantum numbers between 200 and 400 (electrons with higher principal quantum numbers are ionized by stray electric fields and do not contribute to the PFI signal), the collision rate is lo8- 109/s. Therefore, this collision between the Rydberg electron and the ionic core could provide a quenching mechanism.
J. Phys. Chem., Vol. 99,No. 6,1995 1641 Depending on the energy level difference between the X state and the A state, two cases need to be considered. The first case is where the energy level of the A state is higher than that of the X state. This case corresponds to the rotational levels with J+ = 7/2-11/2. For transitions directly to the Rydberg states of the A state, as a result of this collision, the ionic core relaxes to the X state, while the Rydberg electron takes up the extra energy. If these electrons are originally in those states that are less than 0.7 cm-’ below the ionization threshold, they will be ionized after the collision. Rydberg electrons with the ionic core in the “2Z+” state, either directly formed from excitation or transferred from the A state through collision, are still long lived, and these electrons contribute to the PFI-ZEKE signal. For the rotational energy levels with J+ 5 5/2 or J 1 1312 the rotational energy of the A state is lower than the corresponding level of the X state. During the electron-core collision, the ionic core can absorb energy from the Rydberg electron to evolve into the X state, by analogy with the relaxation behavior found for J+ = 13/2 in the previously quoted LIF experiments. If the Rydberg electrons are originally at the lower end of the Stark-ionized range, after this energy loss their principal quantum numbers will be so low that they cannot be field ionized. In either case, the collision between the Rydberg electron and the ionic core decreases the population of the Rydberg electrons converging to the A state, and the detected signal corresponds to ionization of the Rydberg electrons from the X state. However, this argument mainly applies to those states with relatively strong perturbations. For the energy levels with J+ = 1/2 and 3/2, the efficiency of this coupling process should be much smaller. As pointed out by Dentamaro et al.,25 population transfer between the unperturbed states was negligible. On the basis of this analysis, it is easy to understand that the measured PFI-ZEKE signal for transitions to the J+ 1 5/2 levels, for example, the 3/2(1), 5/2(1), 5/2(2), and 7/2(0) lines, should correspond to ionization of Rydberg electrons from the X state. The -1/2(1) transition is with J+ = 1/2, therefore it should correspond to ionization of the electrons from the A state. The 1/2 branch and the 5/2(0) or 3/2(2) line need further examination. For the 5/2(0) or 3/2(2) line, it is not possible to make a clear assignment, as can be seen in Table 2, so we cannot be certain that the pattem of observing “X’ state thresholds is followed, although it is a reasonable conclusion from the spectrum. The 1/2 branch does not follow our model, however, as the observed branch is best assigned as an overlap of the 1/2(0), 1/2(l), and 1/2(2) lines, with only the 1/2(2) line corresponding to a strongly perturbed ion core (I+ = 5/2). We can propose no fully satisfactory explanation for this, other than to note that ab initio calculations for the PFI-ZEKE spectrum of the CO+ X 2Z+state show the 1/2 branch to be d ~ m i n a n t . Since ~ this theoretical prediction disagrees with e~periment,~ and since the “X’ state lines would gain intensity by the small amount of mixing with the “A” state, this explanation is not air-tight. Another interesting effect in the perturbed spectrum is the strength of the 5/2(0) and 5/2( 1) lines, relative to the same lines in the 2J11/2 (v+ = 1) spectrum (see Figure 4). A possible source of enhancement is complex resonances with the state that can be seen in the PIE spectrum. Therefore, to fully understand this PFI-ZEKE spectrum, interactions between at least four channels need to be considered: the ionization continuum, the two Rydberg series converging to “22+”and “2111/2”, and the interloper with v’ = 3, and n = 5 converging to A *lIv+ = 3 (Figure 1). Finally, we note that the A 211 and X 2Z+states can also be
1642 J. Phys. Chem., Vol. 99, No. 6, 1995
mixed through the electric dipole moment, p, and the electric field, E. However, for the spectra reported here, with p = 0.2 eaoZ7and E = 0.8 Vlcm, the interaction Hamiltonian, HA-x= Ep = 0.7 x cm-’, is still negligible. Field-induced transitions have been observed in the threshold photoelectron spectra of H20, where the effect of the external field on the nonzero quadrupole moment was used to explain the observed extra rotational lines.28
4. Conclusion The unusual rotational structure of the A 2 1 1 ~ , 2 v+ = 0 PFIZEKE spectrum is explained in terms of collisional quenching of the ionic core by the Rydberg electron. The efficiency of this coupling process is closely related to the extent of perturbation in the electronic wave function of the ionic core. For transitions with J+ 2 512, strong perturbation was reported, and autoionization of the Rydberg electrons initially formed with the ionic core of *lI1,2 results in the absence of the rotational transitions to these states. For ionic states with J+ < 312, this effect is negligible, and the -112(1) transition to the A state was observed. Some aspects of this mechanism still need refinement, for example, the explanation for the 1/2 branch is still not completely satisfactory. In addition, the intensity of the 512 branches is stronger than expected, and the effect of resonant enhancement needs to be quantitatively evaluated in order to fully understand the underlying dynamics. Improved resolution in the PFI-ZEKE spectrum of this state would improve the certainty of our current spectral assignments and possibly lead to a better understanding of the effect of perturbation of the PFI-ZEKE line intensities. For states without perturbation, the PFI-ZEKE spectra can be explained using the BOS model. The similarity in the fitting coefficients between the two spin-orbit components is consistent with the basic assumption of the BOS model. The relative intensity distributions of the two components are more complicated, and no simple explanation is offered at the moment. The signal intensities corresponding to different ionic vibrational states do not follow the Franck-Condon integrals. Acknowledgment. This work was funded by the Natural Sciences and Engineering Research Council of Canada (NSERC) and by the Network of Centres of Excellence in Molecular and Interfacial Dynamics (CEMAID), administered by NSERC. W.K. thanks NSERC for a graduate scholarship. References and Notes (1) Muller-Dethlefs, K.; Schlag, E. W. Ann. Rev. Phys. Chem. 1991, 42, 109.
Kong and Hepbum (2) Habenicht, W.; Riesler, G.; Muller-Dethlefs, K. J. Chem. Phys. 1991, 95, 4809.
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