Autoionizing resonance in photoionization from the 1π u level of acetyleneKoichiro Mitsuke and Hideo Hattori Citation: The Journal of Chemical Physics 102, 5288 (1995); doi: 10.1063/1.469254 View online: http://dx.doi.org/10.1063/1.469254 View Table of Contents: http://scitation.aip.org/content/aip/journal/jcp/102/13?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Studies on autoionization states of Sm by 3-step resonance photoionization AIP Conf. Proc. 454, 257 (1998); 10.1063/1.57195 Photoionization of the 3σ and 1π orbitals of CH J. Chem. Phys. 92, 536 (1990); 10.1063/1.458456 Resonances in molecular photoionization. II. Autoionization effects in the NO molecule J. Chem. Phys. 87, 331 (1987); 10.1063/1.453632 Autoionizing Effects on the Photoionization Cross Sections of Be, Mg, and Ca from Pseudowavefunctions J. Chem. Phys. 56, 666 (1972); 10.1063/1.1676923 Absorption and Photoionization Coefficients of Acetylene, Propyne, and 1Butyne J. Chem. Phys. 40, 558 (1964); 10.1063/1.1725154

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Autoionizing resonance in photoionization from the 1 pu level of acetyleneKoichiro Mitsuke and Hideo HattoriDepartment of Vacuum UV Photoscience, Institute for Molecular Science, Okazaki 444, Japan

~Received 4 October 1994; accepted 21 December 1994!

Autoionizing resonance of acetylene is studied by photoelectron spectroscopy using synchrotronradiation. Pronounced vibrational excitation in the C–H stretching moden1 is observed in the(1pu)

21X 2Pu band of C2H21 at a restricted photon energy range from 12.8 to 14.1 eV. It is

concluded that the 3sg→3su autoionizing transition at;13.3 eV gives rise to an anomalouslybroad maximum in the~1pu!

21 photoionization cross section curve. The strongn1 excitation isexplained as that the equilibrium C–H bond length differs from the neutral and ionic ground statesto the (3sg)

21(3su)1 resonance state. Constant ionic state spectra for thev153 and 4 levels of the

X 2Pu state measured over the same energy region show fine structures with regular spacingscorresponding to the vibrational levels of the (3sg)

21(3su)1 state. ©1995 American Institute of

Physics.

I. INTRODUCTION

Autoionizing and shape resonances have become ofmain interest in the study of molecular photoionizationprocesses.1–4Autoionization arises from interaction betweenionization continua and a superexcited state produced byvalence–Rydberg or intravalence excitation. Dynamics ofautoionization has been understood in terms of the involvedpotential energy surfaces and the electronic coupling be-tween discrete and continuum states. On the other hand,shape resonance occurs when an excited electron is tempo-rarily trapped in a centrifugal barrier which is effective to aparticular l component of a particular ionization channel.Here, l is the orbital angular momentum of molecular wavefunctions.

Acetylene is one of the most vigorously studied poly-atomic molecules from the theoretical aspect of resonancephenomena in photoionization, since C2H2 is isoelectronic toN2 whose photoionization cross section is greatly in-fluenced bysu ~l53! and pg ~l52! shape resonances.1,3

This molecule in its electronic ground state hasD`h

symmetry and the electron configuration is(1sg)

2(1su)2(2sg)

2(2su)2(3sg)

2(1pu)4. Another reason

why C2H2 has attracted considerable attention is that thecross section for photoionization of the 1pu level shows aconspicuous double-bump structure at photon energies of13–16 eV.3–12Peak maxima are located at;13.3 and;15.3eV. This rather stimulative structure has promoted close co-operation between experiment and theory in order to gaininsight into the dynamics of resonant photoionization ofC2H2. Several groups measured photoelectron spectra for theC2H2

1(X 2Pu) band using synchrotron radiation6,7,9,10 anddiscrete line sources.8 At photon energies below 16 eV, theyreported non-Franck–Condon behavior in the vibrational dis-tribution among thev250–2 levels of the C–C stretchingmode n2,

6–10 and excitation of bending vibration inaddition.10 Theoretically, the cross section curve cannot bereproduced by an independent-particle approximation aslong as resonance effect is not taken into account.7,9,13–16

These pieces of work suggest that at least one resonanceunderlies in this region.

For the resonance centered at 15.3 eV, the existence of2su→1pg intravalence transition is unanimouslyaccepted.3,7,9,11–16In contrast, there has been divergence ofopinion as to the nature of an underlying resonance at;13.3eV: a local maximum in the 1pu→edg continuum,

7,13 shaperesonance,14 autoionizing resonance,9,11or some combinationof such resonances.15,16 This discordance is considered todevelop mainly from an ambiguity concerning vibrationalexcitation caused by resonant photoionization, since discus-sion has been made on the basis of imperfectly resolved pho-toelectron spectra~FWHM*120 meV!. In the present study,we perform photoelectron spectroscopy at;13.3 eV withrelatively high resolution to get evidence in favor of a spe-cific resonance. The C2H2

1(X 2Pu) band reveals selectiveexcitation in a single vibrational mode which is by no meansexcited through direct ionization. This proves that the3sg→3su autoionizing resonance contributes definitively tophotoionization from the 1pu level of C2H2.

II. EXPERIMENTAL METHODS

Details will be published in a forthcoming paper, so onlyessential parts of experiments are described here. All mea-surements are carried out at the UVSOR Synchrotron Radia-tion Facility in Okazaki using a 3 mnormal incidence mono-chromator equipped with a concave 1200 lines/mm grating.The spectral resolution is set to;1.5 Å ~21 meV at 13.3 eV!,corresponding to a slit width of 500mm. A sample gas ofC2H2 is expanded from a 50mm sonic nozzle. A molecularbeam intersects with the monochromatized photon beam at90° at a photoionization region of 0.533 mm2. The incidentlight is estimated to be about 80%–90% linearly polarizedalong the axis of the molecular beam. Photoelectrons aresampled through a hole of 3 mm diameter in the perpendicu-lar direction to axes of both molecular and photon beams.They are focused onto a 2 mmentrance hole of a doublefocusing electrostatic energy analyzer~Comstock AC-902!which is made of two copper concentric 160° spherical sec-tor surfaces. The mean radius of the electron orbit is 54.7mm. For most of the present results, the energy resolution ofa photoelectron spectrum is estimated to be about 60 meV

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~FWHM! from the bandwidth of the Ar1 ~2P1/2,3/2! states.Transmission efficiency of the analyzer is corrected by tak-ing a photoelectron spectrum of O2 at a wavelength of 584 Åand comparing with the calibrated spectrum reported bySamson and Gardner.17

III. RESULTS AND DISCUSSION

Figure 1~a! shows a photoelectron spectrum of theX 2Pu band of C2H2

1 at a photoexcitation energyEhn of 15.3eV. Observed peaks with spacings of 0.22 eV are assigned tothe vibrational ground state andv251,2 states of the C–Cstretching moden2.

6–10 Similar distributions are obtained atEhn in the energy range from 14.1 to 21 eV. The vibrationalbranching ratios are in fair agreements with those predictedfrom Franck–Condon factors between the neutral and ionicground states, although a little deviation is observed belowEhn;16 eV probably because of the effect of the 2su→1pg

autoionizing transition.7,9

BelowEhn;14.1 eV theX 2Pu band drastically changesand undergoes extensive vibrational excitation. The spectrumin Fig. 1~b! at Ehn513.3 eV, e.g., contains two progressions0.22360.004 eV apart,P1 andP2; both of them show aseries of equally spaced peaks with average spacing of0.37660.006 eV~3030650 cm21!. We measure a photoelec-tron spectrum of C2D2 atEhn513.3 eV in order to assign thishighly excited vibrational mode. The C2D2 spectrum in Fig.1~c! also shows two progressions 0.20360.004 eV apart,P3and P4, but average spacing is markedly reduced to

0.30260.010 eV ~2440680 cm21! by deuteration. Hence,progressionsP1–P4 are likely to be in the vibrational modeclosely connected with the motion of H or D atoms. Reuttet al. measured high-resolution HeI photoelectron spectrafor C2H2

1(X 2Pu) and C2D21(X 2Pu).

18 They reported vibra-tional frequencies ofn251829 cm21, n45837 cm21, andn5,1100 cm21 for C2H2

1 , and n152572 cm21, n251651cm21, n45702 cm21, andn5,1000 cm21 for C2D2

1 .19 Here,n1, n4, andn5 denote the totally symmetric C–H stretchingmode, thetrans-bending mode, and thecis-bending mode,respectively. ProgressionsP1–P4 are considered to be inthe n1 mode, judging from a sizable effect of deuteration onthe frequency and good agreement between the average spac-ing of P3 orP4 andn1 for C2D2

1 . We dismiss the possibilityof progressions associated with the degeneraten4 or n5 modebecause the spacing is much larger than two quanta of thebending excitation. It should be noted that excitation of onlythen1 mode occurs inP1 andP3, since the lowest memberfor each progression is the vibrational ground state. On theother hand, the energy shift fromP1 to P2 agrees well withn2 for C2H2

1 and that fromP3 toP4 with n2 for C2D21 . This

suggests thatP2 andP4 are then1 progressions in combi-nation with onen2 quantum.

The v1>2 members inP1–P4 almost disappear below12.8 eV. Namely, the pronouncedn1 excitation is observed ata restricted energy region between 12.8 and 14.1 eV asshown in Fig. 2. This region is compatible with the extent ofthe 13.3 eV resonance in the cross section curve for photo-ionization from the 1pu level.

4–7,11,12It is therefore expectedthat the resonance results from a transition to an autoionizingstate which has equilibrium C–H bond lengths much largerthan those of the neutral ground state. If an autoionizationlifetime is longer than the period of then1 motion, vibra-tional features of the involved state could be resolved in a

FIG. 1. Photoelectron spectra of theX 2Pu band of ~a!,~b! C2H21 and ~c!

C2D21 measured with a resolution of;60 meV~FWHM!. The photoexcita-

tion energy is fixed at~a! 15.3 and~b!,~c! 13.3 eV. The (v1 ,v2) markdenotes the vibrational state in whichv1 andv2 quanta of the C–H~or C–D!and C–C stretching modes, respectively, are simultaneously excited. Theasterisks indicate the vibrational ground state andv251,2 states of then2mode, which owe part of their intensity to direct ionization.

FIG. 2. Photoelectron spectra of theX 2Pu band of C2H21 at the photoex-

citation energy from 12.8 to 14.1 eV measured with a resolution of;60meV ~FWHM!. See the caption of Fig. 1 for an explanation of the (v1 ,v2)mark and the asterisks.

5289K. Mitsuke and H. Hattori: Photoionization of acetylene

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photoionization cross section as a function ofEhn . The 13.3eV resonance has, however, been considered structureless inphotoabsorption and photoionization efficiency curves.4,11,12

Vibrational structures of the autoionizing state are smearedout by uniform background due to direct ionization and pos-sibly have escaped detection, since these curves represent thephotoionization cross section integrated over all vibrationallevels of C2H2

1 . Indeed, there is substantial contribution ofdirect ionization leading to the vibrational ground andv251,2 states as indicated by asterisks in Fig. 2, even in theneighborhood of a resonance maximum.

The consideration above induced us to perform vibra-tionally resolved constant ionic state~CIS! measurements toclarify vibrational structures of the autoionizing state byeliminating the effect of direct ionization. For this purpose,the photon energy and the electron kinetic energy being ana-lyzed are swept synchronously so that the photoionizationcross section to a particular vibrational level ofC2H2

1(X 2Pu) can be monitored. The resolution of the elec-tron energy analyzer is degraded to 110 meV in this measure-ment to obtain enough signal counts of photoelectrons. Fig-ure 3 shows a CIS spectrum with the ionization energy beingfixed to 12.59 eV, i.e., thev153 vibrational level of C2H2

1 .The error bars correspond to two standard deviations derivedfrom the counting statistics. The spectrum shows weak butreproducible structures constituting two progressions0.12560.027 eV apart with average spacing of 0.19560.027eV ~15706220 cm21!. The autoionization lifetime is consid-ered to be much longer than 7310215 s — the shorter limitdeduced from the widths of the vibrational peaks~;0.1 eV!.Similar two progressions are observed in a CIS spectrum forthe v154 level of C2H2

1 .We will discuss the nature of resonance existing around

13.3 eV on the basis of our results together with previous

theoretical studies. Levine and Soven performed a time-dependent local-density approximation and successfully re-produced the double-bump structure in the cross sectioncurve.15,16 From their calculation, the 13.3 eV peak owesabout half of its intensity to the 2su→1pg autoionizingtransition, and the other half to a local maximum in the1pu→epg continuum.3 Inclusion of intrachannel interac-tions, within a random phase approximation, redistributes thenominally discrete 1pu→1pg oscillator strength to higherfrequencies above the ionization potential~511.4 eV! of the1pu level. Lynchet al. assumed the 1pu→1pg shape reso-nance~l52! to be most suitable for the 13.3 eV peak.14 TheirHartree–Fock calculation led to the conclusion that the(1pu)

21(1pg)1 1Su

1 state lies in the ionization continuum.From comparison between panels~a! and~b! of Fig. 1, it

is unlikely that the 13.3 eV resonance is ascribed to the2su→1pg autoionizing transition, since then1 mode excita-tion in the X 2Pu band is indiscernible in a photoelectronspectrum at 15.3 eV where the main band of the 2su→1pg

transition exists. An alternative interpretation for the ob-served n1 excitation is the 1pu→1pg shape resonancewhose centrifugal barrier acting on an excited electron isgreatly affected by internuclear separationrC–H of the C–Hbond. In this case, the electric dipole transition moment isvery sensitive torC–H, even if a range of variations inrC–Hcorresponds to the motion of the ground-state vibration inC2H2.

3 As a consequence, the vibrational distribution of theion differs from that expected from a simple Franck–Condontransition from C2H2(X

1Sg1) to C2H2

1(X 2Pu). Nonethe-less, our results exclude the possibilities that the shape reso-nance is the main contributor to the 13.3 eV resonance. First,high v1 members in Fig. 2 would argue against the(1pu)

21(1pg)1 state. It appears to be unlikely that the shape

resonance leads to the extensiven1 excitation which is com-pletely suppressed in direct ionization. Second, vibrationalprogressions in the CIS spectra for thev153 and 4 levels ofC2H2

1 demonstrate that the underlying resonance is a boundautoionizing state which can be described well with an adia-batic potential energy surface.

We consider the most probable autoionizing resonance at13.3 eV is the 3sg→3su intravalence excitation. The effectof this transition on photoionization cross section is first pro-posed by Hayaishiet al.11 by using the electron-hole poten-tial method, further supported by Parret al.9 Calculationmade by Hayaishiet al.shows that the (3sg)

21(3su)1 state

is bound with respect to the normal coordinate for then1mode.11 Furthermore, the C–H equilibrium bond length isreported to be 1.26 Å — 0.2 Å longer than the neutral andionic ground states and 0.07 Å longer than the(2su)

21(1pg)1 excited state. This equilibrium geometry of

the (3sg)21(3su)

1 state is consistent with the strongn1 ex-citation induced by autoionization. The progressions with av-erage spacing of 0.195 eV in Fig. 3 are probably in then1mode, but definite assignments are still uncertain. This isbecause the vibrational ground state of the (3sg)

21(3su)1

state remains unidentified.Exact vibrational branching ratios for production of

C2H21(X 2Pu) cannot be obtained straightforwardly from the

vibrational distribution in Fig. 2, since detection and energy

FIG. 3. Constant ionic state spectrum with the ionization energy being fixedto thev153 vibrational level of C2H2

1(X 2Pu). The spectral resolution is setto ;1.5 Å ~21 meV at 13.3 eV!, while the electron energy analyzer isoperated with a resolution of;110 meV~FWHM!. Two vibrational progres-sions with average spacing of 0.195 eV may be ascribed to the C–H stretch-ing mode of the (3sg)

21(3su)1 excited valence state.

5290 K. Mitsuke and H. Hattori: Photoionization of acetylene

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analysis is performed for photoelectrons ejected at an angleu590° with respect to the photon polarization direction.There have been cases in which the photoelectron asymme-try parameterb varies from one final vibrational level toanother at autoionizing resonances. An example of this be-havior is given by several members of the autoionizingHopfield series in N2.

20We will attempt in the near future tomeasure the photoelectron spectra for C2H2

1(X 2Pu) at themagic angleu557° ~the polarization of the light;82%!.Thereby, vibrational branching ratios can be compared withFranck–Condon factors between the (3sg)

21(3su)1 state

and the ionic ground state.

ACKNOWLEDGMENTS

Our special thanks are due to the members of the UV-SOR facility for their valuable help during the course of theexperiments. This work was supported by a Grant-in-Aid forScientific Research from the Japanese Ministry of Education,Science, and Culture, Japan~No. 06228231!.

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19One quantum ofn1 stretching excitation in C2D2 could be determined,since the intensity of a Renner–Teller multiplet in combination withn2 isenhanced by a Fermi resonance with thev151 state ~Ref. 18!. ThisRenner–Teller multiplet originates from interaction of thecis-bendingmoden5 and theX 2Pu electronic state.

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5291K. Mitsuke and H. Hattori: Photoionization of acetylene

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