J. Phys. Chem. 1992, 96, 9-12
9
Rotationally Resolved Zero Kinetic Energy Photoelectron Spectroscopy of Nitric Oxide Georg Reiser and Klaus Miiller-Dethlefs* Institut fiir Physikalische und Theoretische Chemie. Technische Universitat Miinchen, Lichtenbergstrasse 4, W-8046 Garching, Germany (Received: September 9, 1991: In Final Form: October 31, 1991)
We report new ZEKE (zero kinetic energy) photoelectron measurements with rotational resolution for 1 + 1' photoionization through the A2Z+ state of nitric oxide. Total angular momentum quantum numbers between 0 and 8 in the intermediate state are selected by different A2Z+ X2111/3.transitions. Results for different polarizations are also reported. The results of relative ZEKE photoelectron intensities indicate a contribution from the coupling of Rydberg states of different rotational quantum number of the ionic core, and alignment effects.
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Introduction The investigation of rotationally resolved photoionization processes is an attractive field since these processes correspond to model systems for unimolecular decomposition processes. Conventional photoelectron spectro~copyl-~ does not allow for a sufficient resolution to select rotational levels in the ion. Only recently, experimental methods have emerged to resolve rotational ionic states upon photoionization of molecules, namely zero kinetic energy (ZEKE) photoelectron spectro~copy.~-'~ Compared to the best resolution obtained for time-of-flight photoelectron spectroscopy (PES) of high-lying rotational states of ZEKE spectroscopy offers a 2-3 orders of magnitude better resolution, down to a few tenths of a wavenumber. This resolution is well sufficient to measure rotationally resolved photoionization spectra at the ionization threshold (Le., for the lowest rotational states of the ion, where rotational energy spacing is smallest) even for larger molecules like benzenelo and ammonia.I2 The rotational propensity rules and intensity distributions in such spectra reveal the ionization dynamics and the coupling of the angular momentum of the ejected electron with the rotational angular momentum of the ion core. ZEKE spectroscopy hence leads to insight into the basic photoionization process itself. The interpretation of rotationally resolved ZEKE spectra (and highly excited Rydberg states) has been successfully carried out by Jungen and co-workers using multichannel quantum defect theory (MQDT).z'*22 Ab initio calculations have been pioneered by the McKoy g r o ~ p *and ~ - ~with ~ computed matrix elements an essentially good agreement between calculated and experimental ZEKE spectra has been obtained for ionization of NO through the A - ~ t a t e .However, ~~ some significant deviations were apparent in that work. For ionizing transitions with a downward transfer in rotational quantum number upon photoionization, Le., AN+ = N+ - NA < 0, the experimentally observed ZEKE signal intensities were found to be considerably stronger than the computed ones. For the upward transitions with AN+ > 0 the deviation between theory and experiment was found to be much less. Also, the angular resolved near-ZEKE measurements through the NO A-state show a pronounced difference to the theoretical prediction for the AN+ = N+ - NA = -1 transition with NA = 1 (Le., the ion left in its lowest rotational state with N+ = 0). For these reasons, new ZEKE photoelectron measurements for the rotational levels NA = 0 to NA = 8 in the intermediate A-state, different A X transitions, and different laser polarizations are presented here thus also taking into account alignment effects in the intermediate state. A recent (lower resolution) ZEKE spectrum for NA = 7 from Kimura and co-workers using the R, A X transition provides a matter of comparison." Experiment The experimental setup used here has already been described by Habenicht et al.12 In brief, the ZEKE method is based on extraction by a delayed electric field pulse.4.8~11~'2,'6~26 There are two different detection schemes for ZEKE measurements.
-
-
Author to whom correspondence should be addressed.
The original concept of ZEKE spectroscopy and discrimination against kinetic (near-ZEKE) electrons4 uses a (field-free) delay period to let kinetic (near-ZEKE) electrons escape. The spatially separated ZEKE electrons are then extracted by the delayed pulsed field and detected after their flight through a drift tube.
(1) Turner, D. W.; Baker, C.; Baker, A. D.; Brundle, C. R. Molecular Photoelectron Spectroscopy; Wiley: London, 1970. (2) Kimura, K.; Katsumata, S.;Achiba, Y.; Yamazaki, T.; Iwata, S. Handbook of HeZ Photoelectron Spectra of Fundamental Organic Molecules; Japan Scientific Societies Press: Tokyo, 1981. (3) Berkowitz, J. Photoabsorption, Photoionization and Photoelectron Spectroscopy; Academic Press: New York, 1979. (4) Miiller-Dethlefs, K.; Sander, M.; Schlag, E. W. Z.Naturforsch. 1984, 39a, 1089. (5) Muller-Dethlefs, K.; Sander, M.; Schlag, E. W. Chem. Phys. Lert. 1984, 112, 291. (6) Miiller-Dethlefs, K.; Sander, M.; Chewter, L. A. In h e r Spectroscopy VZk Hansch, T. W., Shen, Y. R., Eds.; Springer: Berlin, 1985; p 118. (7) Sander, M.; Chewter, L. A.; Muller-Dethlefs, K. Institute of Physics Conference; IOP Publishing Ltd.: London, 1986; Ser. No. 84, Section 5, p 189. (8) Sander, M.; Chewter, L. A.; Miiller-Dethlefs, K.; Schlag, E. W. Phys. Rev. 1987, A 36, 4543. (9) Habenicht, W.; Baumann, R.; Muller-Dethlefs, K.; Schlag, E. W. Ber. Bunsenges. Phys. Chem. 1988, 92, 414. (IO) Chewter, L. A.; Sander, M.; Muller-Dethlefs, K.; Schlag, E. W. J . Chem. Phys. 1987,86, 4737. (1 1) Reiser, G.;Habenicht, W.; Miiller-Dethlefs, K.; Schlag, E. W. Chem. Phys. Lett. 1988, 152, 119. (12) Habenicht, W.; Reiser, G.;Muller-Dethlefs, K. J. Chem. Phys. 1991, 95, 4809. (13) Miiller-Dethlefs, K. J . Chem. Phys. 1991, 95, 4821. (14) Tonkyn, R. G.;Winniczek, J. W.; White, M. G. Chem. Phys. Lett. 1989, 164, 137. (15) Haber, K. S.;Jiang, Y.; Bryant, G.;Lefebvre-Brion, H.; Grant, E. R. Phys. Rev. A , in press. (16) Miiller-Dethlefs, K.; Schlag, E. W. Annu. Rev. Phys. Chem. 1991, 42, 109. (17) Takahashi, M.; Ozeki, H.; Kimura, K. Chem. Phys. Lett. 1991, 181, 255. (18) Wilson, W. G.;Viswanathan, K. S.;Sekreta, E.; Reilly, J. P. J. Phys. Chem. 1984,88, 672. (19) Viswanathan. K. S.;Sekreta, E.; Davidson, E. R.; Reilly, J. P. J . Phys. Chem. 1986, 90, 5078. (20) Allendorf, S.;Leahy, D. J.; Jacobs, D. C.; Zare, R. N. J . Chem. Phys. 1991, 91, 2216. (21) Fredin, S.; Gauyacq, D.; Horani, M.; Jungen, Ch.; Lefevre, G.; Masnou-Seeuws, F. Mol. Phys. 1987, 60, 825. (22) Child, M. S.; Jungen, Ch. J . Chem. Phys. 1990, 93, 7756. (23) Dixit, S. N.; McKoy, V. J . Chem. Phys. 1985, 82, 354. (24) Dixit, S.N . ; Lynch, D. L.; McKoy, V.; Huo, W. M. Phys. Rev. A 1985, 32, 1267. (25) Rudolph, H.; McKoy, V.; Dixit, S. N. J. Chem. Phys. 1989,90,2570. (26) Reiser, G.;Rieger, D.; Muller-Dethlefs, K. Chem. Phys. Lert. 1991, 183. 239.
0022-3654/92/2096-9$03.00/00 1992 American Chemical Society
Letters
10 The Journal of Physical Chemistry, Vol. 96, No. I, 1992 NO+X. v+:o. N+-NO
A , V=O.N,-NO
NO'X. v'zO,
X,V=O;?
In
N ' c NO A, v.0.
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I
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6
N, = O 2
3
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' 30520
"
'
1
"
1
"
'
~
30540
'
/
'
"
1
~
'
30580
30560
30450
30600
30500
30600
30550
N,
-0
:l
I
7
0
I
N, = 7
cn
5
5
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NI
30540
30560
30580
2
n
~
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,
,
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30500
30600
30550 8
W N
I
N, : 2 ~
I,, ,:
30520
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30540
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5
, 3oko
[O-li
Figure 1. Rotationally resolved ZEKE spectra for the intermediate rotational states with N A = 0 to N A = 2 populated by the P, A 6 X transition. Pump and probe laser polarizations are parallel. Wavenumbers in the figure are for the ionizing transition. A, v=O. N-,NO
NO'X. v'=O.N'-NO
Figure 3. Rotationally resolved ZEKE spectra for the intermediate rotational states with N A = 6 to NA = 8 populated by the R,, QZ1A X transition and parallel polarizations.
+
NQ'X,
V+:o.
"-NO
A, v.0,
N, 55
X.V=O; A-X;
30500
30550
30600
N, - L
I
imxj w
30500
30550
30500
-
30600
30550
[cm-'l
Figure 2. Rotationally resolved ZEKE spectra for N A = 3 to N A = 5 populated by the P, A X transition and parallel polarizations.
The second ZEKE detection scheme which was applied in the experiments reported here uses the pulsedfield ionization (PFI)" of highly excited, long-lived Rydberg states with high principal quantum number n, a few cm-' below an eigenstate of the molecular ion. These high-lying Rydberg states are field ionized once the pulsed extraction field is switched on and any free electrons are removed beforehand by a very small dc field present during the delay period. The time-of-flight (TOF) of the field ionized electrons can be tailored by using an extraction pulse of suitable shape.' I Again it should be emphasized that the acronym ZEKE describes the concept of rotationally resolved measurements a t threshold more generally than PFI, because pulsed field ionization does not work for systems with a predissociative ionic core (e.g., for an excited ionic state) or in ph~todetachment,~',~~ where Rydberg states do not exist. A slowly rising extraction pulse, as described in ref 11, was employed here, and electrons from pulsed field ionization were detected by selecting their time-of-flight by the boxcar gate. The laser system consists of two dye lasers (FL 3002) synchronously pumped by an excimer laser (Lambda Physik 1003i). Pulse energies were typically 0.5 mJ for the pump laser (coumarin 2, doubled with BBO) and 1 mJ for the probe laser (DCM, doubled with KDP). (27) Kitsopoulos, T. N.; Waller, I. M.; Loeser, J. G.; Neumark, D. M. Chem. Phys. Lett. 1989, 159, 300. (28) Waller, I. M.; Kitsopoulos, T. N.; Neumark, D. M. J . Phys. Chem. 1990, 94, 2240.
A-X;Rl,Q2
I
1
3
A-X
I
,R21 7
6
a
-
Figure 4. Rotationally resolved ZEKE spectra out of the intermediate rotational state with N A = 5 populated by different A X transitions and parallel polarizations. Intermediate A X transitions are, from the top, PI: Q 1 9P2,:R , , Qzl: and RZ1.
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Results The AZZ+state of nitric oxide was selected as intermediate resonance for the ZEKE measurements reported here. Different transitions to populate certain rotational states in the A-state were selected.29 The A-state is the 3s Rydberg state with a 94% s, 5% d, and 0.2%p contrib~tion.2~ This electronic state shows very small spin-rotation splitting and it is very well described by a Hund's case (b). For the PI transition to populate the intermediate A-state, total angular momentum quantum numbers NA (excluding electronic spin) in the intermediate A-state were selected as N A = 0 to N A = 5 . The ZEKE measurements for photoionization through these rotational states are reported in Figure 1 for N A = 0 to N A = 2 and in Figure 2 for N A = 3 to N A = 5 . Comparing the new results to the previously published results from ref 8, one observes in (29) Herzberg, G. Spectra of Diatomic Molecules. In Molecular Spectra and Molecular Structure; Van Nostrand: Princeton, NJ, 1950; Vol. I.
The Journal of Physical Chemistry, Vol. 96, No. 1 , 1992 11
Letters
TABLE I: Relative Intensities (%) for the Rotrtio~lSelective Ionizing Transitions NO'(V'=O,N') N+ 0 1 2 3 4 5 6 7
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NO A(v=O,NA)' 8
9
10
60.2 60.1
12.6 11.0
24.7 25.4
2.3 3.1
0.2 0.3
12.2 12.3
66.4 67.4
8.7 5.5
11.0 14.7
0.9 0.1
0.8
11.1 6.4 7.8
13.9 12.1 11.0
57.8 58.1 46.1
5.5 6.1 7.4
10.6 15.3 24.1
1.o
1.9 3.8
7.1 4.6 8.1 6.6
10.5 7.3 9.5 7.5
13.0 11.0 13.7 10.8
55.0 56.8 49.3 58.2
5.1 7.7 5.9 6.1
8.3 12.2 12.2 8.6
0.8 0.4 1.2 2.1
0.1
0.1
2.3 2.3 3.2
7.0 5.9 8.5
9.5 7.3 14.1
56.4 54.2 55.3
7.4 7.7 7.9
14.4 19.1 9.5
3.0 3.5 1.1
1.2
7.0 8.7 6.2 8.7 6.3 8.3 10.8 7.0
62.4 60.6 62.9 58.5 63.2 49.7 42.3 63.1
5.8 5.6 6.8 6.7 7.8 7.1 9.3 5.9
9.5 9.7 10.9 9.7 13.4 16.2 17.3 10.8
2.8 6.6 5.6
9.6 12.8 12.2
58.8 62.0 52.7
5.9 4.9 6.4
19.2 7.9 15.8
2.4 1.2 1.9
1.8
10.7
11.4
56.2
6.2
11.2
1.9
>o. 1
3.6 2.0
7.7 6.0
10.2 9.5
62.9 61.2
6.1 7.2
8.8 12.0
0.5
0.2 >o. 1
0.7 0.8 0.6
1.3 0.6
1.9 0.8
13.1 13.7 11.5 14.4 7.3 13.2 14.1 11.1
0.1
1.3 4.5 5.3
1.o
11
1.o
0.6
1.o
1.1 1.4 5.0 3.0 0.8
0.8 2.1
"The A-state was populated via different branches of the A X transition (in N notation). The polarization of the ionizing laser was parallel tt or at right angles t- to the laser pumping the A X transition. The signal was obtained by integrating each peak and subtracting the noise. +
+
Figure 1 (as in ref 8) a decrease in intensity for ionizing transitions that involve a change in rotational quantum number (Le., AN+ # 0) when the rotational quantum number NA goes up. For NA = 4 and 5 one finds about 10-20% of the intensity of the diagonal (AN+ = 0) transition in the nondiagonal (AN+ # 0) transitions. The second observation from the new measurements reported here is a dependence of the nondiagonal ionizing transitions with initial rotational quantum number NA. For example, for NA = 3 (Figure 2) one observes a very strong intensity increase for the rotational transitions with AN+ = N+ - NA = -1 and -3. This effect, Le., the higher intensity for the transitions involving a negatiue AN+ compared to a positive W, is observed in nearly all the rotationally resolved ZEKE measurements. This effect cannot be fully understood from the theoretical work of Rudolph et al.25 Higher rotational quantum numbers in the intermediate state were investigated by using the (in J notation) R1,QZltransition to populate the intermediate rotational states between NA = 6 and NA = 8 (Figure 3). For this A X transition one also observes the general effect that transitions for a downward angular momentum transfer are stronger than the transitions for an upward angular momentum transfer. The question of alignment of the intermediate state and its influence on the observed ZEKE intensities has not yet been experimentally investigated. For this, different A X transitions for the same rotational quantum number in the A-state were chosen, NA = 5 in this case. These results are shown in Figure 4. For the same rotational quantum number in the intermediate A-state, one finds a dependence of the intensity of the ZEKE transitions on the rotational transition that has been chosen for populating the N A = 5 state. The results show interesting alignment effects. For the PI transition, the AN+ = -2 transition is stronger than the AN+ = +2 transition. Going from the P,via
-
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the Q1,PZ1and the R,, QZltransitions to the Rzl transition, this effect inverses and the AN+ = 2 transition now becomes stronger than the AN+ = -2 transition. A similar alignment effect is observed for the AN+ = -1 and AN+ = +I transitions, and also for the AN+ = 3 transitions. The alignment effects involved in these experiments can be interpreted in terms of the treatment presented in ref 13 and the ab initio calculations in refs 24 and 25. This will be given in some forthcoming work. All observed results including the ones observed in Figures 1-4 are reported in Table I. The measurements in Table I show the integrated peak intensities observed in the ZEKE spectra for each rotational transition. For the experiments where cross polarization measurements between the pump and probe photon polarization were carried out, these polarizations are denoted by arrows in the table.
Discussion The new rotationally resolved ZEKE spectra of nitric oxide for rotational angular momentum quantum numbers between NA = 0 and NA = 8 of the A22+state of nitric oxide show two interesting effects. For all angular momentum quantum numbers investigated in the intermediate A-state one observes significant AN+ = N+ - NA transitions with AN+ = f l , f2, and f3. For the PI A X transitions (populating the intermediate state) one always observes a higher intensity for the AN+ < 0 (where possible) compared to the AN+ > 0 transitions. Within the present understanding of the theory of ZEKE spectroscopy, as outlined in ref 13, this can be understood in the following way. Generally, for ionization out of the A-state, the transition probability for the ionizing transition involving no change in angular momentum (i.e., AN+ = 0) is higher than for the Ah"+
+
J. Phys. Chem. 1992, 96, 12-14
12 N*:
1
0
2
3
Diss.
Diss.
I
I
II
1
0
Figure 5. Model invoked for the coupling of Rydberg states with different rotational angular momentum of ion core, leading to intensity gain for the A“ = -1 and A“ = -2 transitions. # 0 transitions that involve a change in angular momentum. An intensity gain for the AN+ < 0 transitions compared to the AN+ > 0 transitions is made possible by the contributions (of strong intensity) from Rydberg states converging to rotational states of the ion core for which N* = NA. These Rydberg states can couple
to Rydberg states of lower rotational quantum number (the equivalent of rotational autoionization) and hence lead to an intensity gain for the AN+ C 0 compared to the AN+ > 0 transitions. The intensity gain for the AN+ C 0 transitions depends on the relative rate of predissociation and “autoionization”, where autoionization denotes the coupling between the different Rydberg states. Figure 5 shows a possible explanation of this coupling. It is apparent that the intensity gain coming from the high transition probability of the AN+ = 0 transition only applies to the AN+ < 0 transitions observed in the ZEKE spectrum.
Conclusion The rotationally resolved ZEKE spectra of nitric oxide for selected rotational quantum numbers NA in the A-state show intensity dependencies that can be explained by a coupling of Rydberg states of different ionic core rotational quantum numbers. The intensity gain for the “downward” ionizing transitions compared to the “upward” ionizing transitions can hence be explained. The observed alignment effects apparent when using different transitions to populate the same intermediate NA state are not so easily interpreted. A full calculation as presented in ref 13, using ab initio matrix elements, has to be carried out to explain this. Acknowledgment. We are grateful to E. W. Schlag (Garching) for his continuous encouragement and his support. We also thank the Commission of the European Communities for support of this research.
Zero Kinetic Energy Photoelectron Spectroscopy of p -Difluorobenzene Dieter Rieger, Georg Reiser, Klaus Miiller-Dethlefs,* and Edward W. Schlag Institut fur Physikalische und Theoretische Chemie, Technische Universitcit Miinchen, Lichtenbergstrasse 4, W-8046 Garching, Germany (Received: September 9, 1991; In Final Form: November 21, 1991)
+
We compare the ZEKE (zero kinetic energy) photoelectron spectrum of p-difluorobenzene, obtained by two-color 1 1’ photon ionization via the SI6’ vibrational state as intermediate resonance, with a recent timeof-flight photoelectron (TOF-PE) spectrum by Sekreta et al. The ZEKE spectrum resolves a large number of fundamental and combination bands of the electronic ground state of the cation, which in the TOF-PE spectrum appear as congested and merged peaks. The assignment of the vibrations observed in the ZEKE spectrum agree well with the assignments obtained for the TOF-PE spectrum. Due to the enhanced resolution of ZEKE spectroscopy (2-3 orders of magnitude compared to photoelectron spectroscopy) improved vibrational frequencies in the cation are derived. The adiabatic ionization energy is determined to 73872 f 3 cm-l.
Introduction The study of molecular ions by photoelectron spectroscopy has been limited severely by the experimental resolution (some 5-10 meV) that can be reached with conventional electron energy In particular, a n a l y ~ e r s or ~ - by ~ the time-of-flight for larger molecular systems involving low-frequency modes and congestion by combination bands, these limitations make it increasingly difficult to observe in detail the vibrational structure of such molecular ions. Supersonic jet cooling reduces cong e s t i ~ n ? and , ~ the combination of time-of-flight PES with resonance-enhanced multiphoton ionization (REMPI-PES or excited state PES) allows to influence the vibrational propensity for photoionization by selecting the intermediate vibrational state. REMPI-PES, however, does not provide a significant improvement as regards the resolution of the photoelectron analyzer itself. A new approach, introduced by Miiller-Dethlefs and coworkers,’*2’ is ZEKE spectroscopy which allows an experimental resolution down to a few tenth of a wavenumber’’ and, hence, in most experiments is close to laser resolution or limited by incomplete state selection in the neutral. *Author to whom correspondence should be addressed.
0022-365419212096-12$03.00/0
ZEKE spectroscopy has been employed to study the rotationally and, resolved photoionization dynamics of molecules’ ‘3’3~’53’620~2’ (1) Sekreta, E.; Visvanathan, K. S.;Reilly, J. P. J . Chem. Phys. 1989, 90, 5349. (2) Turner, D. W.; Baker, C.; Baker, A. D.; Brundle, C. R. Molecular Photoelectron Spectroscopy, Wiley: London, 1970. (3) Kimura, K.; Katsumata, S.; Achiba, Y.; Yamazaki, T.; Iwata, S. Handbook of He1 Photoelectron Spectra of Fundamental Organic Molecules; Japan Scientific Societies Press: Tokyo, 1981. (4) Berkowitz, J. Photoabsorption, Photoionization and Photoelectron Spectroscopy; Academic Press: New York, 1979. (5) Long, S.R.;Meek, J. T.; Reilly, J. P. J . Chem. Phys. 1983, 79, 3206. (6) Conaway, W. E.; Morrison, R. J. S.;Zare, R. N. Chem. Phys. Lett. 1985, 113, 429. (7) Allendorf, W.; h h y , D. J.; Jacobs, D. C.; &re, R. N. J. Chem. Phys. 1989, 91, 2216. (8) Pollard, J. E.; Trevor, D. J.; Lee, Y. T.; Shirley, D. A. Reu. Sci. Instrum. 1981, 52, 1837. (9) Yang, 2.Z.; Wang, L.S.; Lee,Y. T.; Shirley, D. A,; Huang, S.Y.; Lester Jr., W. A. Chem. Phys. Lett. 1990, 171, 9. (IO) Mtiller-Dethlefs,K.; Sander, M.; Schlag, E. W. Z. Naturforsch. 1984, 390. 1089. (1 1) Miiller-Dethlefs. K.; Sander, M.; Schlag, E. W. Chem. Phys. Lett. 1984. 112. 291. (12) Miller-Dethlefs, K.; Sander, M.; Chewter, L. A. in Laser Specrroscopy VII; Springer Verlag: Berlin, 1985; p 118.
0 1992 American Chemical Society
