Volume 73, number 1 CHEMICAL PHYSICS LETTERS 1 July I980

THE He(l) PHOTOELECTRON SPECTRUM OF ATOMIC IODINE

BY PHOTODISSOCEATION OF MOLECULAR IODINE *

Daniel WIRE and T. KOENIG Department of Cirenustry, University of Oregon. Eugene. Oregon 97403. USA

Received 1 February 1980, m final form 31 March 1980

The photoelectron spectrum of atomrc todme has been observed by photodlssociation of molecular lodme with an argon 101-1 Iascr. The relative mtenslty of the lodme atom slgnal IS strongly dependent on the pressure of added zutrogen. The re- subs suggest that photol>s~s IS a feasible altcmatnc means of studymg other transients which IS yet to be uttied.

1. Introduction

The study of the photoelectron spectra of short- hved chemical species is a relatively new and growmg field which can provide valuable mformatlon on the structures of reaction mtermediates. The experimental problem 1s to fmd conltlons under which the concen- tratlon of the transient IS sufficiently high to allow Its detectlon by the low sensltwlty photoelectron tech- nique. The transuxkt concentration m a flow system can be approxrmated by

[T] = PRp exp(-At/T) , (1)

where P IS precursor pressure, [T] IS the transient con- centratlon, Rf is Its rate of formation, Af IS the tune mterval between formation and detection (trme of fight), and 7 (the transient hfetime) IS the reciprocal of the sum of & channels for its destructlorl (r’R,&‘_ Transients have, by defmltlon, “short” hfe:L’I:?s so that Iugh values of Rf are always necessary.

Flash vacuum pyrolysis [l] IS one method which has been previously demonstrated to give concentra- tions of orgaruc radicals [2-I] and unstable [5] hydrocarbons wluch are Hugh enough to allow photo-

* Taken m part from a thesis by Dame1 Imre m part*& fulfii- ment of a Bachelor of Science Degree with dlstmctlon, University of Oregon, 1979.

electron studies. Hugh Rf values can be obtamed by temperature control and suitable design of precursor structures. However, the resolution of the photoelec- tron measurement is vulnerable to any source of space charge 1~1 the photoionlzmg region. This raises

serious practical problems since productlon of surface

deposits IS almost inevitable with pyrolytic methods. A second successful means of generating transients

for photoelectron studies IS mlcrowave chscharge. Halogen atoms [6,7] and species such as SO [8], O&A,) [9],ClO’ [IO] and CF, [l I] are examples. Dyke et al., 1121 have used secondary reactions of fluonne atoms to generate NH2 and HS radicals. The microwave method suffers from problems with field effects in the discharge Itself so that the generatmg source must be relatively remote from the photo- lonlzation regon. The time of fhght (At) becomes long and, unless I- is also long, the concentration of the desired species is reduced. The lack of selectivity of microwave dscharge II-I generatmg polyatomlc transients severely hmits its applicabhty.

In the present communication, we report the first example of the photoelectron spectrum of a transient using photolysls as the generating reactlon. The par- ticular reaction is the phototisociatlon of lodme mol- ecules to gwe observable quantities of lodme atoms. Th.rs reaction was chosen as an engineering case smce it has been well studed by other methods. We be- heve the results demonstrate the feasibility of produc- mg a variety of moderately short-hved species at rela-

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Volume 73, number 1 CHEMICAL PHYSICS LETTERS 1 Suiy 1980

trvely cool temperatures in close proximity to the pho- toionizatron region. At the lower (than pyrolytic)

temperatures, hfetimes can be longer and the close spacurl proxnmty of generation and detection reduces the tune of Bight at. Both of these factors suggest an increased range of transients accessible to photoelec- tron studres.

2. Experimental

The instrument used here was specially designed to allow experimentatron with vanable generatrng reac- tions and geometries. The samplmg system 1s shown schematrcally m fig. 1. The electron velocrty analyzer was a 127” cylindrical eIectrostatrc sector. Electrons were detected usmg a GahIeo Electra-Optrcs channel- tron and after amphficatron read through a drgrtal- analog converter. Sector and recorder sweeps were also generated by d&al-analog devrces. The photo- romzation chamber consisted of a 0.5 rnch stamless- steel tubular section wrth a 0.006 mch aperture sht and was located 5 mm from the entrance of the elec- trostatic sector. Under these conditions, 20000 counts/s at 25 meV fwhm on argon peaks were ob- tainable.

The principal photolysis arrangement 1s shown in fig. 2. It consrsted of two concentric tubes, the outermost containing a relatively high pressure of iodine and inert gases whrch effused into the photo- iomzation region through a 0.012 inch pinhole. The mnermost tube was quartz and provided a free path for the photolytic radiation. The vapor pressure of

Fg. 1. Schematic of the samphng system for a photoelectron spectrometer. PI, dIffuslon pump for main chamber, P,, dL fusion pump for sample flow. D, hquid mtrogen trap.

LlQUlO Na TRAP

I II

FREE LIGHT PATH

MANOMETER

lNLET

Fg. 2. Photolysls cell for photoelectron detection of atomrc lodme.

Iodine between room temperature and ==SO”C was sufficient to give 200-2500 counts/s, depending on mert-gas pressure.

The photolytic light source was a Spectra-Physics 17 l-l 8 cw argon Ion laser which was operated in the all wavelength mode at 5-10 W output power. The same laser was used on the 4880 A and 5 140 A single bnes wrth outputs of 2 W.

3. Results and discussion

Fig. 3 displays our best results which were obtained using the laser in its all wavelength mode * with the total power output of 9 W. The ratio of iodine atoms to molecular iodme increased by a factor of four with the addition of 26 Torr of nitrogen. The spectra are similar to those obtained by de L.eeuw et al. [7] by secondary reactions of atomic chlorine. Our spectra show some improvement m resolution and signal-to- noise which is particularly evident in the bands as- signed to the 3Po and 3Pt states of the atomic iodine cation. The measured ionization potentials (table I) are in perfect agreement with those of Moore 1131, within the limits of our accuracy. The intensity ratios of the 3P2,tp lines of the spectrum were estimated by the approximate areas under the observed peaks (table 1). These estimates were not very merent

* The approximate power distriiution in the all-line made of operation of this laser is 38% in the three A > 4998 A iines (5287 A. 5145 A and 5017 A) and 62% m the A < 4998 A lmes (4965 A, 4880 A, 4765 A, 4727 A, 4658 A, 4.519 A and 4545 A).

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Volume 73, number 1 CHEMICAL PHYSICS LETTERS 1 July 1980

\A,45C-E I nm(9W) 1

c

L-

~

W

z a

t +N2 (26 mm WI

9 IO I1 12 13 14 9 IO II 12 13 14

IP (eV)

Fg. 3. photoelectron spectra of atomtc todiie.

associated Franck-Condon envelope (fig. 4). Scheme I summanzes the features of these mecharusms wluch are essentnd for the present expenments- Excitation by hght of wavelength shorter than 499 run produces

I;(3”,t-J +)u above rts drssociatron hmrt as shown schematically by fig. 4 1141. Spontaneous (Jr2 fast) drssociatron mto one ground-state (2P3j2) and one exerted-state (*P& iodine atom then occurs- The lifetune of the *PI,* atoms 1s too short for detectron

from the mtensrties expected from the degeneracles of the states. The transrtron to the IS, state of atomic rodme catron was not detected-

The radratron from the laser m rts all-lme mode al- lows for productton of rodme atoms from both of the mechanisms * which can operate by absorptron mto the first (311Uo + + ‘Z> electronic transrtron and its

* The power output IS dtstrtbuted between exntatron mto the banded (A > 4998 a) and continutm regrons (& < 4998 18) regions of the absorptton spectrum.

eV

Ftg. 4. Schematrc representatron of the potential energy curves and electromc absorpttons (A = 4500-6000 A) of molecular rodme.

Table 1 Photoelectron iomzatton potenttals and mtensttres of todme atoms (*Pa/a)

Iontc state IP (ev) Relatrve uttenstty

t.hrs work ref. [7] ref. [13]

3P* 10 45 10.43 10 45 1.0 3Po 11.25 11.23 11.25 0.2 3P, 11.33 11.30 11.33 0.4 ‘D; 12.15 12.13 12.15

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Volume 73, number 1 CHEMICAL PHYSICS LETTERS 1 July 1S80

h>499 nm mechanism

Scheme I

I h<499nm mechanism

01 I;(311uo+)b ->

I

k,(-Rvl)

ksb4 ‘I kz

1 k6 t2)lji q k3 1

I;(%I”()-)ll - -2

312 - I*(2p1,2)

by the present method (k3 fast), so that for our pur- poses the X < 499 nm mechanism gives two 2P3,2 atoms per quantum of absorptron. The rate of forma- tion of the atomic spectes (Rf) is sunply Q?s (which rs proportronal to the laser power and the absorption co- efficient) times I2 pressure,

Rf(.X < 499 nm) = 2QJI2] . (2)

Absorptron of light of wavelength longer than -99 run [15] produces 15 wluch is bound (311uo +)b and, m the absence of perturbation, this specres de- cays by fluorescence (k7 of scheme I). However, col- hsrons (ks [Ml) quench the fluorescence by transfer [16] to the 3ll,u- potentral (fig. 4) which 1s not bound. Spontaneous (ks fast) formation of two 2P3/3 atoms then occurs. The rate of for-matron Rf of the atoms by thrs mechanism is

Rfjh > 499 run) = a1 [1212$ IM]/Qs [Ml + k7) , (3)

where %‘1 1s proportronal to the powers and the ab- sorptron coefficients for the long wavelength laser lmes. The hfetime of the atoms (‘P3/2) is determined by the rate of unimolecular diffusion to the wall (k+[wall] I [MI) and the rate of termolecular combma- tion of 21’ and a foreign molecule (M). The drffuston rate is inversely proportional to the pressure so that 7 can be expressed as:

WI r = k, [wall] + kg @4] 2 [I’]

[Ml = k, [wall] - (4)

The pressure dependence for steady-state I’ concen- tration with the h < 499 run mechanism has been clearly demonstrated by Harada and Mori [ 171. They used the 4880 A line of an argon ion laser for produc- tron and electronic absorption (1830 A) for detechon.

Their results and the comments of de L.eeuw (71 ccn- cerning wall preparation, both indicate that the kq term dominates in the pressure range available to us (OS-26 Torr).

The ratio of atomic to molecular iodine from the all wavelength mode of laser operation according to scheme I is finally gtven by

where B represents the constant time of fIight (LU) factor (e-*r/‘).

The increase rn the [r’]/[I,] ratio with added foreign gas is predrcted to be first or second order in CM] depending on the value of k7 relative to kg and as relative to *I. Attempts were made to study the h < 499 and X > 499 run mechanisms separately. Iodine atoms were observed using both the 4880 A and 5 140 A single laser lines at powers of -2 W. The [I’]/[I,] ratio increased with added nitrogen in both cases. The qualitatrve results suggest that a1 is com- parable to a, and, at the highest pressures (26 Torr) of added inert gas, k5 [M] is larger than k7.

In our view, the main significance of the present work is rn demonstrating the feasibility of using cont- mercrally available photolytic sources to produce s&f- ficient quantrtres of transtents to allow photoelectron detection. With a suttably designed photolysis cham- ber tt should be posstble to excite stable substances havmg only moderately strong electronic absorption (E = 1000) m sufficient number density to allow photoelectron detection of triplets and other organic intermediates such as carbenes and nitrenes. We are presently attempting to extend this method to such systems.

Acknowledgement

We are grateful to the National Science Foundation for financial support. It is also a pleasure to a&now& edge the contributions of Professor CE. KIopfenstein, Larry Sims, David Akey and Ted Hinke for the design and construction of the instrument. We are also in- debted to Spectra-Physics Corp. and Mr. Robert Yazell for the loan of the laser used here. The fellow- ship support (2979-80) of the John Simon Cuggen-

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Volume 73. number 1 CHEMICAL PHYSICS LETTERS 1 July 1980

heun Memoml Foundation IS also gratefully acknowl- edged.

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