Ion rotational distributions for single‐photon VUV photoionization of the 1b_1 orbital of the X̃ ^1A_1 ground state of the jet‐cooled water are reported. These spectra reveal significant type a transitions which are seen to arise from odd angular momentum components of the photoelectron matrix element. The resulting photoionization dynamics are quite nonatomic‐like

Grant, E. R.

Lee, M.-T.

McKoy, V.

Tonkyn, R. G.

Wang, Kwanghsi

White, Michael G.

Wiedmann, R. T.

Caltech Authors - Main

Ion rotational distributions for near‐threshold photoionization of H2O
M.‐T. Lee, Kwanghsi Wang, V. McKoy, R. G. Tonkyn, R. T. Wiedmann, E. R. Grant, and Michael G. White 
 
Citation: The Journal of Chemical Physics 96, 7848 (1992); doi: 10.1063/1.462381 
View online: http://dx.doi.org/10.1063/1.462381 
View Table of Contents: http://scitation.aip.org/content/aip/journal/jcp/96/10?ver=pdfcov 
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LETTERS TO THE EDITOR 
The Letters to the Editor section is divided into four categories entitled Communications, Notes, Comments, and Errata. 
Communications are limited to three and one half journal pages, and Notes, Comments, and Errata are limited to one and 
three-fourthsjournal pages as described in the Announcement in the 1 January 1992 issue. 
COMMUNICATIONS 
Ion rotational distributions for near-threshold photoionization of H20 
M.-T. Lee,a) Kwanghsi Wang, and V. McKoy 
Arthur Amos Noyes Laboratory of Chemical Physics, b) California Institute of Technology, Pasadena, 
California 91125 
R. G. Tonkyn,c) R. T. Wiedmann, E. R. Grant,d) and Michael G. White 
Chemistry Department, Brookhaven National Laboratory, Upton, New York 11973 
(Received 5 February 1992; accepted 4 March 1992) 
In a recent paper, we reported on the rotationally re-
solved threshold photoionization of H 20 into the (000), 
( 100), and (010) vibrational levels of the i 2 B 1 ground 
ionic state of H 20 + .1 High resolution (- 1 cm -I) thresh-
old photoelectron spectra for both room temperature and 
jet-cooled (15 K) H 20 were obtained by the pulsed field 
ionization (PFI) method in conjunction with a coherent 
VUV radiation source. The spectra could be readily as-
signed to two types of rotational photoionization transi-
tions, corresponding to specific changes in the asymmetric 
top angular momentum projection quantum numbers Ka 
and Kc' Most of the stronger lines can be classified as type 
c rotational transitions (DJ(c = 0, DJ(a = ± 1) but type a 
(DJ(c = ± 1, tlKa = 0) transitions are also clearly evident, 
particularly in the jet-cooled spectra. The appearance of 
type a transitions with tlKa = 0 is in variance with the 
predictions of a multichannel quantum defect theory 
(MQDT) analysis of photoionization of asymmetric top 
molecules by Child and Jungen.2 These authors predict 
that photoionization will only involve a subset of type c 
transitions with I t::.Ka I = I Ka+ - K~ I = 1 and 
I t::.N I = IN + - Nj I.;;; 1, where N is the total angular mo-
mentum exclusive of spin. These limits on the changes of 
core angular momenta arise from the assumption that the 
Ib l molecular orbital can be described exclusively in terms 
of an atomic Py orbital with both I and its body fixed pro-
jection I A I equal to one. Although the authors character-
ize these as propensity rules, they are predicted to become 
stronger as the upper state approaches the Hund's case (d) 
limit, i.e., as the Rydberg levels converge to the ionization 
threshold. More recently, Gilbert and Child3 have pre-
sented a model based on field-induced autoionization in an 
effort to explain the appearance of type a transitions in the 
threshold spectra of H 20. Although such rotational auto-
ionization processes could give rise to the appearance of 
rotational final states which are nominally forbidden by 
optical selection rules, the specific application to H 20 also 
invokes an atomic like description of photoexcitation from 
the Ib l molecular orbital. 
In this work, we report the results of an ab initio cal-
culation of the near-threshold photoelectron spectrum of 
water into the i 2BI (000) ground ionic state. In this cal-
culation, no assumptions concerning the atomic character 
of the initial and final states are made and the continuum is 
calculated in the full anisotropic potential of the ion core. 
Both type a and type c rotational photoionization transi-
tions are predicted and the intensities are in excellent 
agreement with the observed PFI results. Analysis of the 
partial wave distribution shows that type a transitions oc-
cur with the ejection of nearly pure P wave continua at 
near-threshold energy. These calculations emphasize the 
importance of the nonspherical nature of the molecular ion 
potential which can scatter the photoelectron into various 
angular momentum components. 
The threshold photoelectron spectrum of the i 2 B 1 
(000) state of H 20 + presented here was obtained by the 
PFI method which is a variant of the zero-kinetic-energy 
(ZEKE) technique developed by Muller-Dethlefs, Schlag, 
and co-workers.4 The PFI technique takes advantage of the 
continuity of oscillator strength for absorption into Ryd-
berg levels 1-2 cm- I below an ionization threshold and for 
photoionization just above that threshold.5,6 Tunable nar-
rowband (0.7 cm- I ) VUV radiation in the range 98.3-
98.1 nm required for one-photon photoionization into the 
(000) level was produced by third harmonic generation of 
UV light in a free-jet expansion of argon gas. The overall 
linewidth of -2 cm- I (FWHM) is a convolution of the 
VUV bandwidth and the Stark shift induced by the pulsed 
electric field (0.3 V fcm). Rotationally cold water was pre-
pared in a skimmed molecular beam formed from the ex-
pansion of a 200 Torr mixture of 10% water in argon 
through a pulsed valve with a 0.5 mm nozzle. A complete 
account of the experimental details can be found 
elsewhere. 1.7 
In our derivation of the rotational selection rules for 
photoionization of water, we have used Hund's case (b) to 
represent the ground states of both H 20 and H 20 + . The 
molecular z axis is chosen to be the C2 symmetry axis and 
the x axis is in the plane of the molecule (or ion). Thus, 
the body-fixed axes x, y, and z coincide with the a, c, and 
b axes, respectively. With these choices, it can be shown 
that the bound-free dipole matrix element can be written 
as8 
7848 J. Chern. Phys. 96 (10), 15 May 1992 0021-9606/92/107848-04$006.00 © 1992 American Institute of Physics 
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Letters to the Editor 7849 
[(~;+ 
~; ~:)l, (1) 
with !l.N = N + -Nv and !l.p = p + -Pi- In Eq. (1) Cis 
related to the electronic transition dipole moment given in 
Ref. 8, N + and Ni are the total angular momenta (exclu-
sive of spin) for the ion and ground state H20, respec-
tively, K + and Ki their projections on the z axis, p + and 
Pi the parities of their rotational wave functions, Nt the 
angular momentum transfer, I an angular momentum com-
ponent of the photoelectron matrix element, A. its projec-
tion along the molecular z axis, and f..L the light polarization 
index in the molecular frame. With the symmetry proper-
ties of the asymmetric top 9 and our choice of the molecular 
axes, it can be shown that !l.Ka is even (odd) when !l.N 
+ !l.p is even (odd). Thus, using this relationship and Eq. 
( I) we obtain the selection rules 
!l.Ka + I = odd, (2) 
and 
(3) 
with !l.Ka = K/ -K~ and !l.Kb = Kt -K~ where Ka and 
Kb are the projections of the total angular momentum (ex-
cept spin) along the a and b axes, respectively. In C2v 
symmetry,9 the x, y, and z components of the dipole mo-
ment operator belong to the b2• b l • and al irreducible rep-
resentations, respectively. There are three corresponding 
dipole-allowed continuum channels ka2' kal' and kbl for 
photoionization of an electron from the Ib l orbital of the 
ground state H20. leading to the i 2BI ground state of 
H20 + . In this case. even A. and odd f..L are associated with 
the kal and ka2 channels, whereas odd A. and even f..L are 
associated with the kb l channel. Therefore A. + f..L is always 
odd for all allowed transitions. Thus. from Eq. (3) we 
obtain 
(4) 
From Tables II and IV of Ref. 9, it can be shown that 
!l.Ka + !l.Kc is even (odd) when!l.Kb is even (odd). Hence, 
we have 
(5) 
Equations (2) and (5) show that both type c (!l.Ka = odd 
and !l.Kc = even) transitions. which are associated with 
even I partial waves. and type a (!l.Ka = even and 
!l.Kc = odd) transitions, which are associated with odd Ps, 
are allowed. However, type b transitions are forbidden. 
In our calculations of the rotationally resolved 
threshold photoelectron spectra for H20, the ground 
state wave function is obtained at the Hartree-Fock level 
at the known equilibrium geometry (RO-H = 1.8085 a.u. 
and eH-O-H = 104.50°).10 For the final state, we use the 
frozen-core Hartree-Fock approximation, in which the 
ion-core orbitals are assumed to be identical to those of 
the initial state of H20 and the photoelectron orbitals are 
obtained as solutions of the static-exchange Hamiltonian of 
the ionY These photoelectron (continuum) orbitals are 
obtained numerically using the iterative Schwinger varia-
tional methody,13 Furthermore, we assumed that no spin 
exchange takes place during the supersonic expansion of an 
equilibrated water sample. Therefore a ratio of 3:1 is as-
sumed for ortho to para water. Boltzmann distributions 
are calculated separately for the ortho and para species at 
15 K. 
Figure I shows results of ab initio calculations of the 
rotationally resolved threshold photoelectron spectra along 
with the measured PFI spectra for the i 2 B I «()()() ground 
state of H20 + . A photoelectron kinetic energy of 50 meV 
is assumed in the calculation. The calculated spectra are 
convoluted with a Gaussian detection function having a 
full width at half-maximum of 2 cm -I. The agreement 
between the calculated and experimental spectra is very 
encouraging. These spectra show both type a and type c 
transitions in contrast to the prediction of Child and 
Jungen2 of type c transitions only. Type a transitions are 
due to odd angular momentum components of the photo-
electron matrix element which arise primarily from the 
torques exerted by the molecular ion potential. Since the 
Ibl orbital of the i IA I ground state of H20 has almost 
purep (99.7%) character, predominant even partial wave 
contributions to the photoelectron matrix element, i.e., 
2p -+ ks, and kd transitions, leading to type c transitions, 
are expected in an atomiclike picture. Interestingly, close 
comparison of calculated vs experimental intensities shows 
a slightly systematic asymmetry between rotational transi-
tions with !l.N = -I ("P" branch) and !l.N = + I ("R" 
branch). Similar effects in which anomalous intensity ap-
pears in photoionizing transitions with negative changes in 
core angular momentum have been seen in many other 
cases. 14,15 Its source has been traced in several definitive 
cases to a rotational coupling mechanism similar to that 
proposed by Gilbert and Child.3,15 Thus some evidence for 
such coupling can be detected in the rotationally resolved 
photoionization spectra of H20 for both type a and type c 
transitions. However, the present work clearly shows that 
it is not an essential feature for the appearance of type a 
transitions. 
Figure 2 shows the photoelectron angular distributions 
for the specific rotational ion levels shown in Fig. 1. Fol-
lowing the selection rule of Eq. (2), the angular distribu-
tions for type a transitions (1 01 -+000, 000 -+ 101 , 101 -+202• 
and 111 -+ 212 ) have almost pure p wave contributions to the 
photoelectron matrix element. Interestingly, several of the 
type c transitions 110-+000.101-+111' and 110-+220 have al-
most pure s wave contributions whereas the recent model 
calculations of Child and Jungen2 and Gilbert and Child3 
using atomic analogies consider exclusively d wave final 
states for type c transitions. The 101 -+211• 000 -+212' and 
101 -+ 313 transitions have about equal sand d wave contri-
butions. 
These calculations emphasize the importance of the 
nonspherical nature of the molecular ion potential which 
J. Chern. Phys., Vol. 96, No. 10, 15 May 1992 
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7850 Letters to the Editor 
CD 
>-.... 
.... 
«S 
.... 
CD 
~ 
Experiment 
0.0 
101700 101800 101900 
>. 
.... 
.... 
III 
c:: 
CD 
.... 
c:: 
...... 
1.0 
101800 101900 
Photon Energy (em-I) 
FIG.!. Experimental (top) and calculated (bottom) ion rotational distributions for photoionization of the Ib l orbital of the i IAI ground state ofHp. 
The a and c labels indicate type a and type c transitions, respectively. 
can scatter the photoelectron into angular momentum 
channels that differ significantly from expectations based 
on atomiclike selection rules. Such nonatomic-like behav-
ior of the molecular photoelectron matrix element has been 
extensively documented in previous experimental and the-
oretical studies involving resonance enhanced multiphoton 
ionization of excited Rydberg states of diatomic 
molecules. 16 
Work at the California Institute of Technology was 
supported by grants from the National Science Founda-
tion, Air Force Office of Scientific Research, and the Office 
110 ..... °00 (c) 1 .. -0 .. (a) lOI-ttt (c) 0 .. -1 .. lal Ito-2oe c 101 -.201: ~a~ lu-2t. a 
0 ex) 0 ex) ex) 
0 .. -1" leI 110-211 a 1"-2,, (c) 0 .. -2,. (c) 1,,-2 .. (c) 1"-3,, (c) 
0 0 ex) 0 ex) 
FIG. 2. The calculated photoelectron angular distributions associated 
with specific rotational levels of Fig. 1. The a and c labels indicate type a 
and type c transitions, respectively. e = 0" is vertical. 
of Health and Environmental Research of the U.S. Depart-
ment of Energy. We also acknowledge the use of the re-
sources of the Jet Propulsion Laboratory /Caltech Cray Y-
MP2E/116 supercomputer. One of us (M.T.L.) thanks the 
Conselho Nacional de Desenvolvimento Cientlfico e Tec-
nologico (CNPq, Brasil) for financial support. Work at 
Brookhaven National Laboratory was supported by the 
U.S. Department of Energy, Office of Basic Energy Sci-
ences under Contract No. DE-AC02-76CHOOO16. E.R.G. 
acknowledges support from the National Science Founda-
tion (Grant No. CHE-8920555). 
a)Pennanent address: Departamento de Quimica, Universidade Federal 
de sao Carlos, Caixa Postal 676, CEP 13560 sao Carlos, sao Paulo, 
Brasil. 
b)Contribution No. 8572. 
c)Present address: Materials and Chemical Sciences Center, Pacific 
Northwest Laboratory, Richland, Washington. 
d) Department of Chemistry, Purdue University, West Lafayette, Indiana 
47907. 
I R. G. Tonkyn, R. T. Wiedmann, E. R. Grant, and M. G. White, J. 
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2M. S. Child and Ch. Jungen, J. Chern. Phys. 93, 7756 (1990). 
3R. D. Gilbert and M. S. Child, Chern. Phys. Lett. 187, 153 (1991). 
4K. Muller·Dethlefs and E. W. Schlag, Annu. Rev. Phys. Chern. 42, 109 
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6R. G. Tonkyn, J. W. Winniczek, and M. G. White, Chern. Phys. Lett. 
164, 137 (1989). 
7R. G. Tonkyn and M. G. White, Rev. Sci. Instrum. 60, 1245 (1989). 
J. Chern. Phys., Vol. 96, No. 10, 15 May 1992  This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to  IP:
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Letters to the Editor 7851 
SM._T. Lee, K. Wang, V. McKoy and L. E. Machado (to be published). 
9R. S. Mulliken, Phys. Rev. 59, 873 (1941). 
lOG. Herzberg, Molecular Spectra and Molecular Structure IlL Electronic 
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(1982). 
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A. P. Lima, J. Chern. Phys. 89, 2998 (1988). 
13L. E. Machado, L. M. Brescansin, M. A. P. Lima, M. Braunstein, and 
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14R. T. Wiedmann, R. G. Tonkyn, E. R. Grant, and M. G. White, J. 
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ISR. G. Tonkyn, R. T. Wiedmann, and M. G. White, J. Chern. Phys. (in 
press). 
16See, for example, S. N. Dixit, D. L. Lynch, and V. McKoy, Phys. Rev. 
A 32, 1267 (1985); H. Rudolph, V. McKoy, and S. N. Dixit, J. Chern. 
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R. N. Zare, ibid. 91, 2216 (1989); E. de Beer, C. A. de Lange, J. A. 
Stephens, K. Wang, and V. McKoy, ibid. 95, 714 (1991); K. Wang, J. 
A. Stephens, and V. McKoy, ibid. 95, 6456 (1991); K. Wang and V. 
McKoy, ibid. 95,4977 (1991); 95, 7872 (1991); 95,8718 (1991). 
J. Chern. Phys., Vol. 96, No. 10, 15 May 1992 
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Ion rotational distributions for near-threshold photoionization of H_2O

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Ion rotational distributions for near-threshold photoionization of H_2O

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