Journal of Electron Spectroscopy and Related Phenomena, 15 (1979) 269-274 0 ElsevlerScientlflc PubllshlngCompany,Amsterchun-PnntedmTheNetherlsnds

269

VACUUM UV PHOTOELECTRON INTENSITl OF GASEOUS COMPOUNDS

I He1 SPECTRA OF SIMPLE COMPOUNDS

K KIMUKA, Y ACHIBA, M MORISHITA and T YAMAZAKI

Physical Chemistry Laboratory, Institute of Applied Electricity, Hokkaido University, Sapporo 060, Japan

ABSTRACT

A new method for determining absolute photoionization cross sections for gaseous

compounds is proposed In this method a mixture of a sample and a standard gas (NZ)

is used in photoelectron intensity measurements so that the relative intensity of the

component is obtained with respect to NZ. The relative photoelectron band area is

converted to the absolute photoionization cross section on the basis of the absolute

cross-section data of N Z recently reported by Samson et al This method has been

applied to various aliphatfc compounds to study the effect of alkyl substitution on

photoionization cross sections of O- and N-nonbonding electrons for 584-i radiation

INTRODUCTION

Quantitative measurements of photoelectron intensity by 584-i He1 or any other

resonance radiations have important meanings in physical and analytical aspects If

relative photoelectron intensities are determined for any gaseous compounds with

respective to a reference sample, the quantitative character of photoelectron spectra

will be much more enhanced On the other hand, photoelectron band areas are closely

associated with partial and total photoionization cross sections which are important

for testing theoretical models of photoionization process The differential photo-

ionization cross section a'(8) = do/da for producing photoelectrons in the solid angle

dC at the angle 9 for unpolarized light is theoretically related to the total photo-

ionization cross section o of producing a specific ionic state by the form (refs l-3)

u'(0) = (a/4lT){l- (8/4)(3cos% - 1)) (1)

where 9 is the asymmetry parameter

Although there have been a considerable number of experimental studies on relative

photoelectron intensities within molecule for various compounds in vacuum uv photo-

electron spectroscopy (refs 4-6), there have so far been no studies on relative

intensities among different compounds or absolute specific photoionization cross

sections except for several simple compounds which have recently been studied in

270 K KIMURA etsl

detail as a function of photon energy with the use of electron energy analyzers

that have been corrected for electron transmission (refs 7-11)

The present paper is the first report of a series of our systematic photoelectron

intensity studies of gaseous organic compounds The main purposes of this work are

1) to establish a simple method of determining absolute photoionization cross section

using a standard gas in a sample gas with a certain mole fraction, and 2) to study

effects of alkyl substitution on the photoionization cross sections of oxygen- and

nitrogen-nonbonding orbitals in aliphatic compounds using a He1 resonance source

EXPERIMENTAL

Photoelectron measurements were carried out with a spectrometer with a hemispheri-

cal electrostatic analyzer of 10 cm in diameter, using a He1 resonance source. The

spectrometer is essentially the same as used previously (refs. 12, 13). The resolution

is about 30 meV as measured for Ar (FWHM) using 584-i radiation For the present

purpose a sample reservoir system and a pressure measuring system were attached to

the spectrometer The sample reservoir system consists of three 2-R glass bulbs in

which mixtures of the sample and the standard are filled with different mole ratios

at a total pressure of Ca 50 Torr Nitrogen was used as a standard sample throughout

this work The typical mole ratios are 2 1, 1 1 and 1 2 Each mixture was left at

least one day before use The pressure measuring system consists of an MKS Baratron

pressure gauge and an Edwards pirani gauge which were used to determine the actual

mole ratio of the sample to the standard in the ionization chamber of the spectro-

meter A Nupro variable leak valve was used to controle the sample pressure of the

ionization chamber (Ca 1~10~~ Torr) against a reservoir pressure (Ca 50 Torr).

For each mixture the spectrum was repeatedly measured for several times, the * count rate being stored in a multichannel analyzer (16 bit, 4K memory) at an energy

interval of 2 meV Corrections for electron collecting efficiency were carried out

on the basis of the intensity data of N2, O2 and CO2 reported by Gardner and Samson

(ref 14) Both the corrections for analyzer transmission and mole fraction were

carried out with a computer system (YHP 2105 A) connected to the multichannel analyzer

After the intensity corrections, both the peak height and the area of photoelectron

band for each compound were obtained with respect to the standard molecule The first

peak of N2 (at 15.60 eV) was taken as a standard in the peak-height determination

The relative value of the band area proportional to 0'~ was converted to its

absolute value by assuming that the (T'L of the N2 first band is 0.78 + 0.05 Mb

which is derived from Eq (1) with u = 8 4 f 0 3 Mb and 8 = 0 68 f 0 05 reported

by Samson et al (refs 10, 15) Here, 0'~ means a'(8 = 90')

The compounds studied are very simple compounds (Co, 02, H20. NH3, CH4) and O-

and N-containing aliphatlc compounds (shown in Table 2).

UPSOFGASEOUSCOMPOUNDS 271

RESULTS and DISCUSSION

Results obtained here for the differential photoionization cross section o'I and

the partial photoionization cross section (J for the several simple compounds are

summarized in Table 1, and those for the nonbonding electrons in the various 0- and

N-containing aliphatic compounds are in Table 2 The o values of this work in

Tables 1 and 2 were obtained from our values of o'r using Eq (1) with available

asymmetry parameters In the cases that there are no available data for the asymmetry

parameter, the assumed values were used in obtaining the o values in Table 2

The reproducibility of a series of several runs for each mixture was within 3 %

The standard error introduced in the transmission correction is 4 % in the region

of ionization energy below 19 eV, while it is 15 % above 19 eV

The principal advantage of the present method may be that the photoelectron inten-

sities of the sample and the standard can be determined simultaneously under the same

experimental conditions However the mole fraction of the sample in the ionization

chamber must be known and it is not always the same as in the sample reservoirs

In order to check this we have studied the mole fraction of the sample in the ioniza-

tion chamber by a combination of both the Baratron and pirani gauges Under our

experimental conditions it has been found that in the mixtures of N2-Ar, N2-CO2 and

N2-acetone there are essentially no differences in the mole fraction between the

ionization chamber and the sample reservoir In the mixtures of nitrogen with much

heavier compounds such as CC14, however, a slight difference in the mole fraction has

been detected Therefore it should be important to determine the mole fraction of the

sample with care It has also been confirmed that the photoelectron intensity is

proportional to the partial pressure of the component Such linearity is important

to determine the relative intensity of the component with respect to the standard

Previously Betteridge et al (ref 20) have indicated that the spectrum of a mixture

is a linear combination of the spectra of the components of that mixture

For N2, CO and O2 the photoionization cross sections have already been determined

in detail as a function of photon energy by Samson et al (refs 8-10) by a combina-

tion of photoelectron spectroscopy and absorption measurements Therefore our

purpose of studying these simple compounds is only to test our method by comparing

the results with other reported ones It is seen from Table 1 that good agreements

are obtained between the two methods The present method therefore may be adequate

for determining 0'~ and CT in absolute values for many other compounds The photo-

electron measurements of Samson et al (refs 8-10) have been carried out at magic

angle (54"44'). Our method may of course be applied to magic-angle measurements,

although the present work was made at 90"

Brian et al (refs 16-19) have recently determined the photoionization cross

sections of several simple compounds using an electron impact ionization technique

which uses coincidence detection of scattered and ejected electrons resulting from

ionization under experimental conditions that dipole transitions dominate The

272 K KIMURA etal

TABLE1

Differential photoionization cross sections ~'l(Mb) and partial photoionization cross section u obtained in Mb units (1 Mb - lo-l8 cm*) for several simple compounds at 584-d radiation, compared with literatures

Partial photoionization cross section o (Mb) Ionic U'l(Mb) Others state This work PES + Abe Electron impacti

N2 X (0 78 + 0.05)a (8 4 + 0 3)a 84?03b 7 9e A 1.09 f 0 04 125*06 12 6 + 0 3b 12 3e B 0 27 f 0.01 2.5 f 0 1 2.1 * o 1b 1.7e

co X 0 79 f 0.04 80205 76503' 8 l= A 1 35 f 0.06 157+09 13 4 f 0 3c 13 7e B 0 24 + 0.03 28*04 2 7+01c 2.1=

02 X 0 55 f 0 02 7.4 f 0 4 70+02d a+A 0 73 + 0 03 84206 82+02d

b 0 43 + 0 02 47+03 5.2 f 0 2d B 0.18 -+ 0 03 18204 30+01d

E2C Bl 0.59 + 0 03 59*04 6 pf A1 0.51 f 0 02 60204 5 gf B2 0 65 + 0 03 84kO8

NH3 Al 0 78 f 0 05 81+06 5 7g El 1.99 f 0 12 23 7219 22 9g

CH4 T2 2 95 f 0 14 3222 22 3l.lh

aTaken as the standard bref 10 Cref 8 dref 9 eref 16, Energy loss E = 21 eV fref 17, E = 22 5 eV gref 18, E - 21 eV h ref 19,E=21eV iconverted from the reported values of the oscillator strength (df/dE) (eV_') by a(Mb) = l.O975xlO*(df/dE)

results of the electron impact method are also compared in Table 1, from which it is

seen that agreements between the electron-impact data and ours are generally good.

Previously, Blake and Carver (ref 21) have also carried out photoionization cross

-section measurements using photoelectron spectroscopy, reporting curves of the

photoionization cross section as a function of the incident photon energy.

For oxygen- and nitrogen-containing aliphatic compounds, so far there have been

reported no data on photoionization cross sections of specific ionic states As far as

the first ionization bands due to nonbonding electrons are concerned, it seems to be

correct that the photoionization cross sectfons reflect mainly substitution effects,

since the ionization energies are close to one another. The o's of the alkyl alcohols

and amines are plotted against the number of carbon atoms in Fig 1, indicating inter-

esting variations In the series of the alcohols, the values of U'L and u increase

with Increasing number of carbon atoms The variation of the photoionization cross

section of the nonbonding electrons may be interpreted in terms of the orbital

interactions between the nonbonding orbital8 and other molecular orbitals It is also

interesting to note that the partial photoionization cross section of the carbonyl

oxygen of acetaldehyde is much smaller than that of the methanol oxygen.

UPSOFGASEOUSCOMPOUNDS 273

TABLE 2

Differential photoionization cross section 0'1 and partial photoionization cross section u (in Mb units) for the 0- and N-nonbonding electrons of aliphatic compounds at 584-i radiation. (S values used in the calculations of o are also shown )

Compound I( E(eV)g U'1(Hb) a(Hb) B

Hz0 12 62b 8 59 0 59 f 0 03 59204 10+01i CH30H 10 94= 10 27 104 f 0 07 ll6+09 0 5 * 0 osj C2H50H 10 64c 10 57 119 f 0 06 135?08 0 44 f 0 033 n-C3H70H 10 49= 10 72 1 41 + 0 10 15 7h O.Sk (CH3120 10 04d 11 17 1 51 f 0 06 16.gh 0 Sk CHgCiiO (C2H 120 10 9 26d 63d 11 10 58 95 0 1 44 84 f f 0.07 0 05 16 9 lh 4h 0 0.5k 5k

(CH3)2CC 9 70d 11 51 1 00 f 0 05 11 2h 0 5k CH3(C2H5)CO 9 56d 11 65 111 f 0 08 12.4h 0 Sk NH3 10 85e 10 36 0 78 f 0 05 8 1 f 0.6 0 82 * 0 lR :;fi:ii, ; . 4;: 11 11 57 71 ; :: i t : if 146208 135+07 0 0 93 84 f. r 0 0.043 05J

n-C3H7NH2 9.44c 11 77 1 28 + 0 07 13 4h 0 gk I-C3H7NH2 9 31= 11 90 1 25 + 0 05 13.lh 0.8k (CH3)2NH 8 97f 12 24 1 62 + 0.07 16 gh 0 gk (CH313N 8 44f 12 77 1 87 + 0 16 19 gh 0 8k a Vertical ionization b ref fief 25

energy 22 'ref 23. dref 12 e ref 24 gPhotoelectron kinetic energy hDerived from the assumed B value

iref 26 jref 27. kAssumed %ef 15

Finally it should be mentioned that Kemeny et al (refs 28, 29) have previously

measured relative o's in the noble gases with respect to a reference (Ar) by uv

photoelectron spectroscopy using a method of mixing the reference and any other noble

gas Absolute C-S'S thus obtained from these relative values are in good agreement

with those obtained from total photoabsorption measurements

0 1 2 3 n-+

Fig 1 Variation of the photoionization cross sections of nonbonding electrons, with increasing number of carbon atoms in aliphatic alcohols and smines at 584-A radiation

274 K KIMURA et al

ACKNOWLEDGEMENT

We are grateful to Dr T Kobayashi of The Institute of Physical and Chemical

Research for sending us the 8 values of methanol, ethanol, methylamine and

ethylamine before publication

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