Rotationally resolved photoionization of polyatomic hydrides: CH3, H2O, H2S, H2CO Ralph T. Wiedmann, Michael G. White, Kwanghsi Wang, and Vincent McKoy Citation: The Journal of Chemical Physics 100, 4738 (1994); doi: 10.1063/1.466264 View online: http://dx.doi.org/10.1063/1.466264 View Table of Contents: http://scitation.aip.org/content/aip/journal/jcp/100/7?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Rotationally resolved photoionization of H2O J. Chem. Phys. 95, 7033 (1991); 10.1063/1.461431 Photoionization of dimeric polyatomic molecules: Proton affinities of H2O and HF J. Chem. Phys. 67, 4235 (1977); 10.1063/1.435404 Magnetic Properties of Polyatomic Molecules. II. Proton Magnetic Shielding Constants in H2O, NH3, CH4, and CH3F J. Chem. Phys. 52, 6411 (1970); 10.1063/1.1672957 Magnetic Properties of Polyatomic Molecules. I. Magnetic Susceptibility of H2O, NH3, CH4, H2O2 J. Chem. Phys. 49, 882 (1968); 10.1063/1.1670155 Ab Initio Calculations of the Barriers to Internal Rotation of CH3CH3, CH3NH2, CH3OH, N2H4, H2O2, and NH2OH J. Chem. Phys. 46, 3941 (1967); 10.1063/1.1840468 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: 131.215.248.200 On: Sat, 17 Oct 2015 06:43:26 Rotationally resolved photoionization of polyatomic hydrides: CH3 , H20, H2S, H2CO Ralph T. Wiedmann and Michael G. White Chemistry Department, Brookhaven National Laboratory, Upton, New York 11973-5000 Kwanghsi Wang and Vincent McKoy Arthur Amos Noyes Laboratory of Chemical Physics, a) California Institute of Technology, Pasadena, California 91125 (Received 19 November 1993; accepted 23 December 1993) Combined theoretical and experimental studies of rotationally resolved photoelectron spectra for single-photon ionization of the outermost valence orbitals of H20, H2S, H2CO, and CH3 are re­ ported. Agreement between calculated and measured spectra is very encouraging. Both show that photo ionization dynamics is very molecular in origin for H20, H2S, and H2CO but quite atomiclike for CH3 . Parity selection rules and the angular momentum composition of the photoelectron are used to illustrate the dynamical aspects of photo ionization of poly atomics as molecular symmetry changes in a group of structurally related systems. I. INTRODUCTION II. THEORY: FORMULATION AND CALCULATIONS Recent studies of rotationally resolved photoionization As noted above, photofragmentation dynamics refers of the nonlinear molecules H20 (Refs. 1-3), H2S (Refs. primarily to the nature and fate of the collision complex which is produced by photoexcitation. Because photoabsorp­ 4-6), and CH3 (methyl radicalf highlight the unique aspects of molecular photoionization as a probe of electron-core in­ tion takes place at short range, the photoionization collision teractions at short range. Specifically, K-state resolution of complex consists of the molecular ion core and a strongly the rotational structure in polyatomic cations allows resolu­ coupled photoexcited electron. The dynamics of this short­ tion of the degenerate photoelectron final angular momentum range collision complex is governed by the usual bound­ i1J states by parity, i.e., I =even or I =odd. As in all photofrag­ bound selection rules governing dipole transitions, e.g., = 0, :!:: 1, as well as other propensity rules which depend on mentation processes, determination of the final state distribu­ specific angular momentum coupling cases. As fragmenta­ tions of the fragments (photoelectron and cation in this case) tion proceeds and the photoelectron moves into the asymp­ provides a dynamical window on the electronic and nuclear totic region, the electron-core interaction becomes less an­ motions of the "collision complex" responsible for the as­ isotropic and the angular momentum coupling becomes more ymptotic fragment distributions. In photo ionization, the col­ appropriate to a photoelectron of well defined orbital angular lision complex corresponds to the photoelectron and ion core momentum and a specific rovibronic level of the ion core. at short range where the anisotropy of the molecular cation This uncoupling can be viewed as a scattering process in potential determines the partial wave distribution that the which the photoexcited electron exchanges energy and angu­ electron will be "scattered" into at long range. The short- to lar momentum with the ion core. The net result is that long-range evolution of the photoelectron leads to spectro­ changes in core angular momentum well beyond !:.J = 0, :!:: 1 scopic properties which are appropriate for I-uncoupled are possible. For absorption of a single photon, conservation Rydberg (high-n) or continuum transitions, but which ulti­ of angular momentum requires only that mately reflect the symmetry of the cation potential. In this work, we present new experimental and theoreti­ cal results for single-photon ionization of the related poly­ i1J=J + -J;=1+3/2,1+ 1/2, ... ,-1-3/2, (1) atomic molecule, H2CO (formaldehyde), as well as theoreti­ cal calculations for the methyl radical whose experimental 7 where I is the orbital angular momentum associated with spectrum has been recently published. Although H20, H2S, ks,kp,kd,'" partial waves and where the photoelectron spin and H2CO are all planar (C 2v) in their neutral and ionic (s=:!:: 1/2) and photon angular momentum (1) result in the ground states, the symmetry of the outermost 2h orbital of z :!::3/2 term. This simple expression highlights the relationship formaldehyde and the assignment of its principal axes leads between the observable rotational branch transitions of the to significant differences in the types of rotational transitions core, given by i1J, and the unobserved partial wave compo­ that are observed. Earlier experimental and theoretical work sition (I) of the outgoing photoelectron. This is a key result for H20 and HzS are also included to illustrate the dynamical and is a significant motivation for pursuing rotationally re­ aspects of photoionization of polyatomics as molecular sym­ solved measurements. In what follows, we develop the theo­ metry changes in a group of structurally related systems. retical basis for interpreting rotationally resolved photoion­ ization with particular emphasis on those aspects which are ')Contribution No. 8888. unique to nonlinear molecules. This article4738 is copyrightedJ. Chern. as Phys. indicated 100 in(7). the 1 Aprilarticle. 1994 Reuse of AIP content0021-9606/94/100(7)/4738/9/$6.00 is subject to the terms at: http://scitation.aip.org/termsconditions. © 1994 American Institute Downloaded of Physics to IP: 131.215.248.200 On: Sat, 17 Oct 2015 06:43:26 Wiedmann at al.: Photoionization of polyatomic hydrides 4739 A. Differential cross section MJ+ level of the ion. An expression for Cfm(MJ;MJ) has 3 Under collision-free conditions, the rotationally resolved been derived by Lee et al. for molecules with C 2u symmetry differential cross section for single-photon ionization of a and has the simple form rotational level of the ground state of a molecule by linearly polarized light can be written as dCT CT dO = 47T [1 + ,8P2(COS 0)], (2) where CT is the total cross section, ,8 the asymmetry param­ eter, and Pz(cos e) the Legendre polynominal. Here the total 2 X _NK++ N·I Nt) + (-l)P+ cross section CT and asymmetry parameter ,8 have the form ,3 [ ( Ki K t 2 PMJMJ.IClm(MJiMJ, , +)1 , (3) 1m X (;: ;: ;:)], (5) and with AN=N + - Ni and Ap= p+ - Pi' In Eq. (5), C is a labo­ ratory frame quantity given in Ref. 3, N is the total angular 5 momentum (exclusive of spin), K is its projection on the z ,8=- axis, P is the parity of the rotational wave function, Ntis the CT /I'm angular momentum transfer, I is an angular momentum com­ ponent of the photoelectron, A is its projection along the molecular z axis, J.L is the light polarization index in the molecular frame, 'Y is one of the irreducible representations (IR) of the molecular point group, q is a component of this I' 2) (I I' 02) (4) -m 0 0 0 representation, and h distinguishes between different bases for the same IR corresponding to the same value of I. where PMJM is the population of the M J. sublevel of the , J, l A similar expression for Clm(MJ;MJ ) of Eq. (5) has ground state and C fm( M JiMJ) is the coefficient for photo- been derived by Wang and McKoy8 for the D3h molecular ionization of the M J. level of the ground state leading to the symmetry group appropriate to CH • It has the simple form I 3 1 Ni Ni , yq ( N t Nt) + (-l)P; ( N+ Nt) Clm(MJiMJ) = 2: C Ihl~/l -K (:;+ t J.L ~)[ Ki K t -K+ -Ki K t Ni Ni + (-l)P+ (N+ Nt) +(-l)P;+P+ (N+ (6) K+ Ki Kt K+ -Ki ;:) ]. A central quantity in these studies is the matrix element IZi~/l for photoejection of an electron from a bound molecu­ lar orbital 4Jr' q' into a photoelectron continuum orbital 'I'~:k)(r). The partial wave components if;~h?;:q of'l'~:k)(r)Aare y'q' y'q' X 4Jih'1 (r)Xh'I'~ (r'», (9) defined by an expansion in generalized harmonics about k of o 0 0 the photoelectron 1/2 'I'(-)(r)= -2) ~ il.I,(-)yq(r)X*yq(k) 9 [,k (7T £..i 'f'khlm him , (7) where !Wm~ is a rotational matrix in Edmonds' notation. 1m Whereas only 1= /' terms are allowed in Eq. (9) for the where X:I~q(k) is a generalized harmonic. Single-center ex­ central fields of atomic systems, where the angular momen­ pansions of if;~h?;:q(r) and 4Jr'q'(r'), e.g., tum of the photoelectron must be conserved, /:/= /' terms arise in Eq.