Vacuum UV photoelectron intensity of gaseous compounds

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

269

VACUUM UV PHOTOELECTRON INTENSITl OF GASEOUS COMPOUNDS I

He1 SPECTRA OF SIMPLE COMPOUNDS

K

KIMUKA,

Physical Hokkaido

ACHIBA, M

Y

MORISHITA and T

YAMAZAKI

Chemistry Laboratory, Institute University, Sapporo 060, Japan

of Applied

Electricity,

ABSTRACT

A new method compounds

for determining

is proposed

absolute

In this method

is used in photoelectron

is obtained with respect

converted

to the absolute

photoionization

aliphatfc

for gaseous

of a sample and a standard so that the relative

gas (NZ)

intensity

of the

band area is

cross section on the basis of the absolute

reported by Samson et al

compounds

cross sections

cross sections

to NZ. The relative photoelectron

photoionization

data of N Z recently

applied to various

a mixture

intensity measurements

component

cross-section

photoionization

This method has been

to study the effect of alkyl substitution

of O- and N-nonbonding

electrons

on

for 584-i radiation

INTRODUCTION Quantitative resonance

measurements

radiations

relative photoelectron respective

intensities

to a reference

sample,

will be much more enhanced associated

with partial

for testing theoretical ionization

of photoelectron

have important meanings

sections

for producing

a specific

intensities

are important

The differential

photo-

in the solid angle

related to the total photo-

ionic state by the form (refs

- 1))

l-3)

(1)

parameter

there have been a considerable

spectroscopy

spectra

band areas are closely

photoelectrons

light is theoretically

If

with

of photoelectron

process

of photoionization

cross section o of producing

photoelectron

character

models

cross section a'(8) = do/da

Although

intensities

the quantitative

aspects

for any gaseous compounds

cross sections which

9 is the asymmetry

electron

are determined

and analytical

On the other hand, photoelectron

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

by 584-i He1 or any other

and total photoionization

dC at the angle 9 for unpolarized ionization

intensity in physical

within molecule

(refs

among different

except for several

number of experimental for various

compounds

studies on relative

in vacuum uv photo-

4-6), there have so far been no studies on relative compounds

or absolute

specific

photoionization

cross

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 hemispherical 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 this work

50 Torr

Nitrogen was used as a standard sample throughout

The typical mole ratios are 2 1, 1 1 and 1 2

least one day before use

Each mixture was left at

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 spectrometer

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'Lof the N2 first band is 0.78 + 0.05 Mb which is derived from Eq by Samson et al

(1) with u = 8 4 f 0 3 Mb and 8 = 0 68 f 0 05 reported

(refs 10, 15)

The compounds studied are

Here, 0'~ means a'(8 = 90')

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

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

UPSOFGASEOUSCOMPOUNDS RESULTS

271

and DISCUSSION

Results

obtained here for the differential

the partial

photoionization

summarized

cross section (J for the several

in Table 1, and those for the nonbonding

N-containing

aliphatic

compounds

Tables 1 and 2 were obtained asymmetry

are in Table 2

from our values

The reproducibility

of a series of several

error introduced

of ionization

advantage

conditions

However

tion chamber by a combination conditions

of both

chamber and the sample reservoir

care

to determine

the relative

intensity

et al

(ref

purpose

of studying

are obtained between for determining

angle

intensity

is

is important

and absorption

ones

the two methods

of that mixture

(refs

measurements

8-10) by a combinaTherefore

our

is only to test our method by comparing

It is seen from Table 1 that good agreements The present method values

of Samson et al

(refs

therefore may be adequate

for many other compounds

The photo-

8-10) have been carried out at magic to magic-angle

measurements,

the present work was made at 90"

Brian et al sections

the mole fraction of the

cross sections have already been determined

(54"44'). Our method may of course be applied

although

in the mole fraction has

that the spectrum of a mixture

energy by Samson et al

0'~ and CT in absolute

electron measurements

the

of nitrogen with much

Such linearity

of the components

these simple compounds

the results with other reported

of N2-Ar, N2-CO2 and

that the photoelectron

20) have indicated

spectroscopy

Under our

of the component with respect to the standard

For N2, CO and O2 the photoionization

tion of photoelectron

to determine

of the component

of the spectra

in detail as a function of photon

under the same

in the mole fraction between

In the mixtures

It has also been confirmed

to the partial pressure

Betteridge

and pirani gauges

a slight difference

it should be important

proportional

inten-

the same as in the sample reservoirs

no differences

such as CC14, however,

is a linear combination

simultaneously

it has been found that in the mixtures

Therefore

3 %

is 4 % in the region

the mole fraction of the sample in the ioniza-

ionization

sample with

correction

the Baratron

there are essentially

been detected

runs for each mixture was within

fraction of the sample in the ionization

N2-acetone

compounds

data for the asymmetry

the o values in Table 2

can be determined

the mole

In order to check this we have studied

experimental

(1) with available

it is 15 % above 19 eV

chamber must be known and it is not always

Previously

of o'r using Eq

of the present method may be that the photoelectron

sities of the sample and the standard experimental

are

in the various 0- and

The o values of this work in

in the transmission

energy below 19 eV, while

The principal

simple compounds

electrons

the assumed values were used in obtaining

The standard

cross section o'I and

In the cases that there are no available

parameters

parameter,

heavier

photoionization

(refs

of several

simple compounds

which uses coincidence ionization

16-19) have recently determined

detection

under experimental

using an electron

of scattered

conditions

the photoionization impact ionization

and ejected

electrons

that dipole transitions

cross technique

resulting

dominate

The

from

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

Ionic state N2

X A B X A B X a+A

co

02

b

E2C

NH3 CH4

B Bl A1 B2 Al El T2

U'l(Mb) (0 78 1.09 0 27 0 79 1 35 0 24 0 55 0 73 0 43 0.18 0.59 0.51 0 65 0 78 1.99 2 95

+ 0.05)a f 0 04 f 0.01 f 0.04 f 0.06 + 0.03 f 0 02 + 0 03 + 0 02 -+0 03 + 0 03 f 0 02 + 0 03 f 0 05 f 0 12 f 0 14

Partial photoionization cross section o (Mb) Others This work Electron impacti PES + Abe (8 4 + 0 3)a 125*06 2.5 f 0 1 80205 157+09 28*04 7.4 f 0 4 84206 47+03 18204 59*04 60204 84kO8 81+06 23 7219 3222 22

84?03b 12 6 + 0 2.1 * o 76503' 13 4 f 0 2 7+01c 70+02d 82+02d 5.2 f 0 30+01d

7 9e 12 3e 1.7e 8 l= 13 7e 2.1=

3b 1b 3c

2d 6 pf 5 gf 5 7g 22 9g 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 h ref 19,E=21eV gref 18, E - 21 eV 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.

273

UPSOFGASEOUSCOMPOUNDS 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(

Hz0 CH30H C2H50H n-C3H70H (CH3120 CHgCiiO (C2H 120

12 10 10 10 10 10 9

(CH3)2CC CH3(C2H5)CO NH3 :;fi:ii, n-C3H7NH2 I-C3H7NH2 (CH3)2NH (CH313N

E(eV)g 62b 94= 64c 49= 04d 26d 63d

8 10 10 10 11 10 11

59 27 57 72 17 95 58

U'1(Hb) 0 59 104 119 1 41 1 51 0 1 44 84

f f f + f f

0 03 0 07 0 06 0 10 0 06 0 05 0.07

59204 ll6+09 135?08 15 7h 16.gh 16 9 lh 4h 11 2h 12.4h

9 70d 9 56d

11 51 11 65

1 00 f 0 05 111 f 0 08

10 85e ; .4;:

10 36 11 57 71

0 78 f 0 05 ; :: t i : if

11 11 12 12

1 1 1 1

9.44c 9 31= 8 97f 8 44f

77 90 24 77

28 25 62 87

a Vertical ionization energy b ref 22 fief 25 gPhotoelectron kinetic energy iref 26 jref 27. kAssumed %ef

+ + + +

a(Hb)

0 07 0 05 0.07 0 16

8 1 f 0.6 146208 135+07 13 4h 13.lh 16 gh 19 gh

B 10+01i 0 5 * 0 osj 0 44 f 0 033 O.Sk 0 Sk 0.5k 0 5k 0 5k 0 Sk 0 82 * 0 lR 0 93 84 r f.0 0.043 05J 0 gk 0.8k 0 gk 0 8k

'ref 23. dref 12 eref 24 hDerived from the assumed B value 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

n-+

2

3

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 etal

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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