Multiphoton ionisation spectroscopy of H2S: a reinvestigation of the 1B1-1A1 band at 139.1 nm

CHEhlICALPHYSICSLETTERS

Volume93, number1

MULTIPHOTON

IONISATION

A REINVESTIGATION M.N.R. ASHFOLD Sclrool of CJmrrcrry Rcccived

1 Seprember

The 3 +

OF THE

t 8, _1 At BAND AT 139.! nm

and R.N. DIXON

1982

1 mulhpholon

1BI

OF H+:

Vnwersrr~ of Bmrol, Brisrol8.WITS Vh’

ionwtron

III both linearly and cwxlarly photon absorption speclrum lion of its

SPECTROSCOPY

(hIPI) speclrum of the ‘61 -‘Al

tr.msitwn III ll$T~ al 139 I nm has been recorded

polarised hght The rotatIonal structure shops marked d~flcrences from rhrt of the oncPropewes of the ewted SLUC revealed through analysts of rlusstrucwrc mcludc confirms-

character, relined values for ~rsA,

B and

C roiatlonnl constanls and the opernuon

of an energy-dcpendcnr

predluoclauon mechanism. It IS shown that the Ilurd-rank tensor component of the tranwon opcrdtor domrnates over lhe first-tank component in this hlPl band. The orbItal nature of this ’ B1 ewlted sta1c IS cnnsldcred.

1. Introduction lnterpretatlon

whether this electronic promotton wolves the populatton of an “s’- or “d’- type Rydberg orbtial rewttt1s of the detailed

form of the electron-

spectrum of H,S has attracted the attention of expenmentahsts [I-6] and theoretictans [7-91 ahke. The current consensuc associates all of the banded features observed III the one.photon absorption spectrum at vacuum-W wavelengths between 160 nm and the first lonisatron potential, 1I8 5 nm [I ,2], with excitation of a 2b l(3p) electron, esscntrally nonbondmg dnd localised on the sulplmr atom, IO d vanety of Rydberg states The vlbratwnal structure of these transitions is dominated by ortgm bands, in heepmg IC

with

the very simdar

structures

for the ground

states

of H,S and H$ [IO]. Several recent slurlIes [I I - I31 have demonstrated the potential of multiphoton iomsatlon (MPI) spcctroscopy as a new and convement means of investigating electromc transitlons to comparattvely long-hved exctted slates, and cspeclally IO Rydberg states The purpose of this communication IS to report the apphcatlon of MPI spectroscopy to a study of the Intense, structured absorptron band of H,S centred at 139.1 nm. The rotattonal structure associated with tins band (the lirst sharp member of Price’s so-called E series [I .I?] but &_Gng

excitation

to a state subse-

quently labelled H I_O]) has been analysed and its asslgnment as ‘Bl * X ‘A, confirwcd [ 141, though 0 009-2614/82/0000-0000/S

02.75 0 1982 North-Holland

debatable

[4,51

In this work, resonant three-photon excrtatlon IO this I Bl s~atc IS followed by further photon absorption, resulling m ion formallon. Spectral interpret+ tion depends upon a knowledge of the rotational lrne strengths for the lnitlal smlultaneous and rate hmltmg three-photon absorption even1 m this molecule It IS Intended that a full descrlptlon of these three-photon rotational hne strengths, together with presentation and analysis of the complete

hlPl spectra

of H$

and

DZS at three-photon energies correspondmg to vacuumUV wavelengths below 160 nm WIII form the basis of longer future pubhcatlons [I 51. Here WC WISII IO demonslrate furlher the capabthtics of thus techmquc, to offer evidence for an energy-dependent predlssocratron m the excited 1B, state and IO comment on IISelecfromc character

2. Experimental A glass hlPl cell of convcntlonal desrgn equipped wtth two. parallel I cm? nickel electrodes (1.5 cm sepnratron) and speccrosll B wmdows set at Brewsrer’s angle was used to contam a static 2 Torr sample of prevtously vacuum dtsttlled H,S (B.D H.. 99.6%)

I

Volume 93. number

CHEMICAL

MPI HI the region between

these plate electrodes

effected

(20 cm focal length

usmg the focused

coated

fused sd~ca lens) output

pumped

tunable

and FL 7-002 respecuvely)

polanser

mtracavity

output

operarmg

hnearly

to ctrcular

a $ wave plate

selected

onented

band were recorded

Incident

Fresnel

cell electrodes 427) (PAR

ion current

l62/

rhomb,

between

spectra

of

UIIS

or the

[ 161. The MPI

note

165) operating

to detectron

of the

by a pyroelectnc

Joulemeter

R,,

was

(Centec

= T’(c

ED

100).

Nreman

[ 171 llas dlscussed the theory

m connection of NH3 general

wtth

for a molecule the interpretztton

We present theory

of the 139 cirauon

I nm band of H,S

by three identical

The mrriv elment state

of frequency

of the more

the diagonal

term

(pomt

group

C,,)

of the translllon

m ex-

operator from

excttatlon

Y and electnc

The first-rank lection

tensor

rule U=

I

where

I

Ir) and I!), wtth homogeneous

dre real contrlbutlons states

’

As in the case of two-photon

hne and band mtensities

widths

to the two virtual are sensrttve

(I)

r, and r,,

mtermedlate

transItIons

[IS]

to the state of

polansatton of the light. Expertmentally the vanatton of polz~nsar~on ‘an be used to ulduce changes m mren-

6

m Imearly,

but

and Bves rise to the se-

However,

the third-rank

polarisatrons,

tensor

and gives nse to the

rule AI = 0, ?I,

9, ?3 The line strength

pendence

on polarisatlon

IS given,

factors,

apart

from

de-

common

by

[ 15,171.

(linear

+ i ITi(B)12)

III linearly

polarisation)

, (3a)

(circular polarisation)

polartsed

_

‘3)

for the third-rank con-

mtenslty

polansed light IS 2.5 times that light of the same mtenslty

This predIctton

IS borne

IL

out for the (weak)

m the 3 + 1 h!PI spectrum

of

NH3 [11,17]. H2.S IS an asymmetrtc near oblate top, so that A’, IS a fairly good quantum number even though the asym-

metry sphtting is resolved. For 3 tB,-IA, tensor

whch

tensor

third-rank analytical symbols

tensor

are identtcal

spectrum;

has two independent rules AK,

The theoretlcal can be evaluated

expresstons [ZO].

factors

one-photon

lead to the selectton

*2 respectively.

transition selection rule

gives the addittonal

= 0, and the hne strength

thtrd-rank I

light,

0, ?I.

m both

those of the type-c

R11= ‘F(E,[

by analogy with fl for

selection

A&

L cdn be

wntten

(I l~~eli~(ilp-rl/~(~l~-El~~ -/tV+lr)(&, -?h+ir)

for three

photons [ 181, We use

contnbutes

polartsed

the first-rank

by lhree

vector

vanish

[ 19).

N, 0, S and T branches

for the mterpretatton

photons

I to state 2 for coherent

photons

symmetry

of MPI spectra

here those aspects

that are necessary

(3)

E ~)-7’~(ll).

tensors

trsnsltion hyperpolansdbdity,

trrbutron m clrculsrly

of three-

of D,,,

T3(t

the symbol B in eq. (2) to sigmfy the tirst molecular

I!(41T3(B)11)

line strengths

lme strengths

spherical

but not for the

product

tions Induced by two tdenkal

TIIUS the theoretical photon

-

R I2 is the sum of one first-

tensor

The zero- and second-rank

f~(&lT3(B)12

3. Three-photon

are both facditated

photons

case -

E E)+(B)+

contributes

by a boxcar

laser output

three colour

rank and one third-rank

not circularly,

III IIS linear gate mode. The

controlled

factors,

in terms of trreductble

For three identical

general

the

(Ketthley

a IO7 amphficauon

eq. (I)

mterpretatron. and the derivation

Identical photons for the same reasons of symmetry that cause the first-rank tensor to vamsh for transi-

at a COJI-

comb arrenuatron

An electrometer

prior

of the mtenstty

monitored

etther

were biased (= 100 V dc) such that

was used to provide

energy

be

This was achieved through

techmcal

tons are collected.

resulting

tensors.

dye laser

m the spectral

of these effects,

line strength

3. An

full details of which WIU be pre-

in a forthcomrng

postttve

MPI

of rotational by expressing

blue hght,

both polarisalrons

controlled

laser output,

sented

cell

light Intensity.

use of a feedback

ewxner

with

with

are valuable

201

This could

by mserlmg

slty whrch

The understanding

(XeCI) EMG

the natural

for operdtlon

the laser and lhe tomsatlon

was AR

on stllbene

polarised

polartsatlon

or an appropriately

stattt

Physlk

ensures that

is verttcally

converted

of an excuser

dye laser (Lambda

19 November 1982

PHYSICS LLTTERS

whereas

to the

components = 0 and A&

line strengths

=

for the

using tabulated

for the appropriate

Wagner 3-j

Volume 93, number 4.

1

CHEMICAL

PHYSICS

Results and discussion

LEITCRS

I9 Novcmbcr I982

Fig. la dlsplnys

the 3 +

I hlPI

spectrum

of H,S

fol-

lowng hnearly polarised laser excttation In the wave. length range 415-420 nm.This band has the sharpest hnes within the overall wavelength range so far studled (390-480

nm), as m the corresponding one-photon

absorptron spectrum [ 1,141, but shows a significantly different rotational structure. Circularly polansed excrtation using laser pulses ofrlre s~n!e e~rgl~ resulted in a spectrum of seemmgly ldentlcal structural appearance, bur Increased m overall mtenslty by a factor of

the dommnnl

- If not the total - contnbutlon

is made

by one, or bolh, of the third-rank tensor componenls to the overall trans!tion probability. The approumste upper and lower state rotational constants due to Gallo and lnnes [ 141 were used m order to slmulatc the spectra arising out of the two thud-rank components. This revealed the evpenmentally observed threephoton spectrum to bc dommated by the thild-ranh tensor component dssoclated with the AK, = 0 selection rule, and enabled assignment of many of the spec-

tral features A Ieat-squares fit mvolvtng 23 of the

MS_

I‘lg I. (a) 3 + len~ly

observdtlons thus mdlcate that three-photon 1B, -1 A I cxcrtatlon

The experlmental

for thus particular

toselher

I MPI spectrum wth

ofHz.5 In the wavelcngh rcpon 4 15-420 nm recorded under conchtlons ot near constant laser I”(b) the bcsr Iit nmul~hon of tbls I B, -‘A, band uun~ the rotntmnnl conrtants and he bro;ldcn,np parm,,-

eters described III the tc\t. 7

Volume 93, number

I

CHEhllCAL

19 November

PHYSICS LETTERS

1982

I I!! 2 Calcul~~rd AKc = 0 tlnrd-rank. tensor component IO lhr 139 1 nm ’ B,-‘Al trrnsrtlon m HzS usmg a hnewdth of 1.5 cm-’ (fwhm) ho = 71897 3 cm-’ ,A’= IO47cm-‘.f?‘=8.91 cm-‘,C’=464cm-‘. ccn~rlugal dlstorlron cocflIlen[s lor the upper SINK rdcntical to those reported for the H~S+(~‘UI) moleLulx ion [?I], ground-slate con%mts from ref [?I 1snd ossummg a IempcrJlure of 300 R The J and A’ struclure xsorrafcd wth the md~r~du.d N, 0, P,Q, R.S and T branches IS drspldycd rn the form of such draprrmls (on .I one half reduced verhcal scale) approprirtlcly positioned above the man spectrum

sharper.

relatrvely

spectrum

unblended,

(u = 0.5 cm-l)

lines m the expenmental

and use of the most recent

ground-stateconslanls [?I] resulted in the following relined state

set of rotatIonal

constants

for the excited

A’= 10.47~005cm-I,B’=S91

C’ = 4.64

f 0 03 cm-*,

two standard efficlents

devlatlons,

tdcntlcal

I B,

the error

and centrifugal

to those for H$‘(z

drstortlon

hne

‘B,)

spectrum

component

ly equdlbrated

for the AK,

of thus transltlon

300

K rotauonal

= 0 third-rank

based on o thermal-

state dlstnburlon

IS

In fig. 2 Clearly, whilst thls calculatron succcssti~lly reproduces the posltions of features having presen(ed

low J’, 11 results

also m a more structured

slve spectrum

than that observed

IS particularly

evident

the Q branch,

which

the low-frequency lower-state 8

I , , -3?,

tal lmewldth

[22]

on the high-frequency 1s predicled

side because

Boltzmann

and exten-

experimentally.

faclors,

This side of

to be stronger of more but which

than

favourable is observed

than those near the

Even the levels of lowest J’

show some prechssociation

co-

the lmes in thr wings of

broader

band centre. A rotationally dependent predlssoclalion III the excited state is proposed m order to account

mental

The computed

Furthermore

hm~ts represent

were assumed. tensor

weaker.

for these dlscrepancres.

+O.Ojcm-1,

where

to be

the band are markedly

at 7 I805 of K!

5 cm-’

laser bandwidth

confirmed IS consistent

Independent

with

an excited-state

Predlssociarlon

MPI

the rate of predlssociatlon loss by predissoclation outslde

the Doppler

hne intensity

(cf.

cm-‘).

the fundalinewidth,

lhs

of the laser intensity,

lifetime

at the three-photon

the observed

+. Secondly,

0 branch

with

respect

of ~2 ps,

the rate of ionisallon ciency

(fwhm).

of
as bang

IO predlssociation affccls

the unblended

1 cm- I dlsplnys an expenmen-

resonant

signal III two ways

level

Firstly

will be In competition

by subsequent decreases where

photons.

the iomsatlon

the lifetime

wilh so that effi-

broadenmg

hmit

- as in this case -

is dlstnbuted

over an increased

IS

the total width,

Volume 93, number with

I

CHChlICAL

a correspondingly

decreased

peak height.

PHYSICS

LCITCRS

Stmula-

I9 November 1981

The fundamental

laser frequency

band profile

were attempted

IS far from

for several

based either

on al’-depen-

gy (ij’-

48000

tremely

diffuse

hfetnlc

of the state, or stns

simple

models

dent or a rotational-energy-dependent

predissoclation

mechanism. The most successful of these assumes a energy

rotatIona

dependence,

lzo and hnewldth spectrum

IQ

of each hne In the theoretrcal

of fig. 2 are modified

11= @Jr, exp(-E:/Eo)

accordrng

mental

spectrum

III the expcrunental hne 331-1,,, spectrum

IS compared

In parhcular

31 7 1999.6

cm-l,

dependence

parabohc

but IS evidently

further

must

tures observed ui lhe one-photon

one of the few MPl component

symmetry

rank tensors

regmn

absorption 130-160

bands for which

only wluch.

to the Tea-

[I-6]

of ihls

The transition at 139.1 nm is

dommates

transitIons

by symmelry.

are required

has been observed [ 151 fur the H$i

nm,assigned transirlon

[S] Io Ihe (Zbl)-‘(rrpb?) ) The simulation

band at I57 9 ‘AZ-x

of rhe profiie

doubt Clearly

regarding

the

the dominance

In the MPI metry

spectrum

spectrum

grounds

the angular

alone,

properfles

shown

assignment.

of the third-rank be explamed

and must

allowed

slate can gcncrale

butlon

to the overall

excltatron

process

a 7ero first-rank three-photon

10 I~IIS

fensor

contli-

translllon

probabd-

ItY

The two, at first sight confhchng, scrv.ILlons - namely that pears strongly

’ BI

in the one-photon

eupenmenlal -I

by the AK,

absorption

= 0 tlurd-rank

tensor

dorm-

component

-

lr3nsLIIon

matrix

element

tween

two or more spherical harmomc contrlbutlons

One posslbihty

IS that

m tl11s Instance, 3da,

orbltals

(I)

spectrum

spectrum

if the rhrce-photon

ofeq.

ob-

band (I) ap-

Al

but (II) shows a three-photon

[I-6], nated

this

leads to an mlerferencc

llus Interference

a moved ?,a!” orb1131

occurs

cnergctlc

rug111 well be mducrd

and a discussIon

&al

of Clzs and D,S

,md

by the non-

or 11s miphca~ions,

served for drscussion m the contell spectra

chxac-

between,

spherical molecular core. Further elaboration model

In the

.nld “d.a,”

Such ,I mlxmg

the comparably

be-

of thrs

wrll bc rc-

of the entlre

hll’l

(1.51.

We are most grateful Research

to the Sclcnce

COUIICI~ for a research

and C. Hill for tlvzlr mv;lluoblc

to h1.R.

GUIISOII

grailt,

.md Cngineermg to K K. Kosser

technrcal

supporl

and

for compu~at~oual XSISIXICP.

on C,, states

References

I II WC 121 WC

component

bc a consequence

of the partlcipatmg

Acknowledgement

IAl

nm [ 141, I, can leave no

in fig.

1B, excited-state cannot

l B,

of the one-

photon absorphon spectrum around 139.1 and of the MPI

level 1s

to be

lhe third-rank component will be AK, = +1_. (In passing, 11 IS worth mentmmng that such characlerlslvz behoviour

dl this two-phoion

of

have first-

In these cases, however, the selection rule for

zero

enhancenienl

and wavelengrhdcpcndcnt,

the third-rank

[ 151 In a molecule

‘A,--‘A,

resonance

Ler for the Rydberg

spectrum nm

In the 3 + 1 MPI spectrum

molecule loo [ 15.23]

C,,

be given

of the maJor

The very shorl

provided by Ihe fact lhdt no scquenhal Iwo-photon

final state through

a

an oversimphfIcatron.

1B, slate. Exh

of H?S m the wavelength

by

through

regwn

[41, contrtbutmg

level Further Justlficdtlon

any posslblc,

can be accommodated

IS not perfect

for tunneling

conslderatlon

of this excited

may be discerned

this

of eq. (4) was Inspired

equations

barrier.

that

than In rhe theoreticdl

3. Even so the agreement

Ihe seml-classlcal

tensor

the experi-

cm-‘.andthefirslSbrdnch

of fig

Finally,

with

at71805.1

The FunctIonal

nature

lo 300 cm-‘.

equivalent

for the much greater prommencc spectrum of the first 0 branch

accounts

I 11-3z1

E,

m fig. lb. Note

spectrum

smlulation

(4b)

ener-

the weak and CY-

i ) falls w&n

first absorpIron

plus one-photon

,

(4cI

energy

cm-

at Ihe two-photon

for neglectmg

,

a characteristic

Thrs broadened

exp(-$/Eo)

cm-l

10 IIIIS IS hkely lo preclude their actmg as real mter-

conrmuum medlales

to (4a)

ah,

exp(Ei/Eo)

w = IQ

lme

the peak-he@1

exp(-.Q&J,

G = &

with

m which

of =:34000

any real state of H?S. The two-photon

tions of the experunental

symof

Ptu. J Clwm Pbys 3 (1936) 147 Prrcc. J P [email protected] and A D \V.IISII Prop Kay Sot

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1

Volume 93. number

CHEMICAL

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LCTTERS

Chem

[VI R Robergeand D R Salahub. J Chem Ph}s 70(1979) 1177 [IO] G Herzberg, Electronic spectra and electronic strwure of polyaromlc molecules (Van Nostrand. Pnnceton. Ill)

PHYSICS

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to bc pubhshed