Photodissociation of zinc diiodide in the gas phase

3. Photochem.

Photobiol. A:

Chem.,

65 (1992)

345

345-353

Photodissociation of zinc diiodide in the gas phase Koji MO& Kazuo Kasatam ‘+, Masahiro Kawasaki++, Eiji Ishitani, Hisanori Shinohara and Hiroyasu Sate*

Makoto

Kobayashi,

Deparhnent of Chemistry for Materials, Faculty of Engineering Mi’e University, Tsu 514 (Japan) (Received

April 25, 1991; accepted December 5, 1991)

Abstract photodissociation of zinc diiodide in the gas phase was studied at 308 and 266 nm. The temporal variation in the density of the ZnI(X 2z) radicals, generated at 308 nm, was monitored by laser-induced fluorescence &IF), and the decay of Zn(4s’ ‘S) atoms, generated at 266 nm, was probed by atomic resonance absorption. The decay of ZnI(X *Q was found to be due to a combination of first- and second-order kinetics, and Zn(4s2 ‘S) atoms decayed by first-order kinetics in our experimental conditioris. When the laser light (308 nm) was focused, emission lines from excited zinc atoms were observed as a result of multiphoton absorption,

The

1. Introduction Many triatomic group IIb metal halides have been the target of intensive research, because the photodissociation of these metal halides can give lasing fragments. After the first report by Schimitschek et al. [l] on the photodissociation of HgBrt at 193 nm to give HgBr(B %+), the compounds HgCl, [2], CdBr, and Cd& [3, 4) and ZnIz [5] were reported to give lasing diatomic metal halide fragments on photodissociation. Lasing from excited atoms, as a result of multiphoton processes, has been reported for Hg12 [6], HgBrz [7], Cd& 14, 6) and ZnIz [6]. The efficient recombination reaction to form the original metal dihalide molecules is the key factor in sustaining lasing action over thousands of pulses. Erlandson and Cool 183 investigated this aspect for the HgBrz system and showed that the fast bimolecular reaction, HgBr + Brz +HgBr% + Br, is responsible for the observed rapid and efficient regeneration of HgBr,, In Cd12, we have shown that the CdI(X %) radicals decay via second-order kinetics [9]. In the present paper, the photodissociation of ZnIz was studied at 308 and 266 nm. The temporal variation in the density of ZnI(X “x) radicals, generated at 308 nm, and Zn(4s2 ‘S) atoms, generated at 266 nm, was measured. The emission from excited zinc atoms, caused by multiphoton excitation at 308 nm, was also investigated.

‘Present address: Department of Chemistry. Fukuoka Women’s University, Kasumigaoka, Higashiku, Fulcuoka 813, Japan. “Present address: Research Institute of Applied Electricity, Hokkaido University, N12 W6, Sapporo 060, Japan. 4Author to whom correspondence should be addressed.

@ 1992 - Elsevier Sequoia.

AlI rights reserved

346 2. Experimental

details

The experimental set-up for the laser-induced fluorescence (LIF) measurement is shown in Fig. 1. Briefly, slightly focused light from a pulsed XeCl laser (Lumonics TE-431T, 308 m-n, approximately 5 mJ pulse-l cm-*) was used to dissociate ZnI, in a quartz sample cell composed of a head and a side-arm. The temperatures of the head and the side-arm were controlled separately. Optical measurements were made on the head of the cell, while the sample pressure was controlled by the temperature of the side-arm. The temperature of the side-arm (T,) was in the range 320-390 “C (this corresponds to Pz,,t2= 0.11-1.03 Torr, 1 Torr133.322 Pa), whereas that of the head (Th) was kept at 500 “C. The output near 662 nm (DCM (4_dicyanomethylene2-methyI-6-@-dimethylaminostyryl)-4H-pyran) dye) of a dye laser (Quanta-Ray, PDL2), driven by the second harmonic of a pulsed Nd:YAG laser (Quanta-Ray, DCR-2, 10 Hz), was frequency doubled using a p-BaB*O., (BBO) crystal, and timed to irradiate the sample at a selected delay time following the dissociation pulse. The delay time between the photodissociation and probe laser pulses was controlled by a home-built pulse generator. The temporal resolution of the delay was 0.4 ps. The LIF from the ZnI(G ‘I&--,X ‘z) bands was monitored using a Nikon G-250 monochromator and a Hamamatsu R928 photomultiplier to measure the temporal variation of ZnI(X *x) density. The resolution of the monochromator was 0.25 nm. For measurement of the atomic absorption and emission of zinc, the thermally controlled cell mentioned above was used. In the time-dependent measurement of zinc absorption, T, and Th were 370-430 “C and 480-510 “C respectively. The fourth (266 nm) harmonic of the pulsed Nd:YAG laser was used to photodissociate Zn12. A Hamamatsu L233-30NQ hollow cathode lamp was used as monitoring lamp. The light signal at 307.6 nm corresponding to the ‘P t ‘S absorption line was isolated with the monochromator-photomultiplier assembly mentioned above. The signal was fed to a transient recorder (Kawasaki Electronica M-50E) with an averager connected to a personal computer. T, and Th were approximately 380 “C and In atomic emission experiments, 500 “C respectively. The XeCl excimer laser was used for photodissociation. The laser light was focused into the cell to cause multiphoton absorption. The signal obtained with the monochromator-photomultiplier assembly was amplified and transferred to a home-built boxcar integrator and recorded on a strip-chart recorder.

Fig. 1. Experimental set-up for the LIF measurement: PG, pulse generator; 0. oven; C, cell; BBO, a &BaB204 crystal; F, filter; TC, temperature controller; W, window; L, lens; MC, monochromator; HV, high voltage supply; PMT, photomultiplier tube; BC, boxcar integrator; COM, computer.

347

3. Results and discussion

The possible photodissociation processes during UV laser irradiation of ZnIz are as follows Z&(X

‘2) = hu -

ZnI(X *2) + I(2P&

E,, = 299 W mol- ’

(1)

-

ZnI(X ‘8) + I*(*P&

EO = 390 W mol-’

(2)

-

Zn(4s* ‘S) + 2I(*P&

E,,=#2

(3)

kJ mol-*

Reaction (1) is possible on energy grounds by one-photon absorption at 308 nm (388 kJ mol-l). Reactions (2) and (3) can be caused by one-photon absorption at 266 nm (450 kJ mol-‘) or by multiple photon absorption at 308 run. The ground and excited energy Ievels by Zn12, ZnI +I and Zn+21 are shown schematically in Fig. 2. 3.1. Temporal behaviour of Z&(X ‘i$) radicals The dispersed fluorescence spectrum, obtained by excitation of ZnI(X %) radicals generated from ZnIz dissociation at 308 nm, is shown in Fig. 3. Frequency-doubled dye laser light (331.7 nm) excites ZnI radicals in the v”=O level through the (0, 0) band in the C, 2II3n+X *s transition. Each band in this spectrum is due to the (0, n) progression of the transition, where n = 1-3. Scattering of the probe beam was essentially negligible. An example of the observed temporal variation in the LIF signal is shown in Fig. 4 for the (0, 0) band at 331.7 nm. The temperature Th was kept at 500 “C. The observed decay can be simulated by a combination of first- and second-order decays - d[ZnI]/&=kr[ZnI]

+k,[ZnI]*

(4)

4'S Zn+ZI

Fig. 2. Energy level diagram of ZnIz, ZnI+I

and Zn+

21.

348

’ ((0.0)

1

1

Adis Aprd

JUL/c.

(0.2)

325

330 335 Wavelength

Fig_ 3. Dispersed

o-o0.00

fluorescence

(0.3)

I

340 / nm spectrum

I

0.05

= 300nm = 33131nm

of ZnI(G

1

0.10

t ime/ms

345

0.15

‘l&-+X

‘2).

A,, -331.7

nm.

I

0.20

Fig. 4. Temporal variation in ZnI(X ‘2) density after the dissociation laser pulse at 500 "C, P zn,2=1.03 Torr. Points: experimental data; full line: simulated curve; upper panel: residuals.

where k, and k2 are the first- and second-order rate constants for ZnI respectively. The rate equation can be solved in a straightforward manner and the time profile of [Zd] is denoted as [ZnI](t) where

= [ZnIJO/{(l +A)&‘--A) [ZnI],

is the initial concentration

(5) of ZnI

and

A = k,[ZnI],/k, is the initial branching ratio between first-order and second-order decay paths. The parameters are decided using a non-linear least-squares method. The full line in Fig. 4 shows the result of the simulation. The residual is plotted in the upper panel. The simulation reproduces the experimenta decay well, except for a small deviation appearing in the long time region. This small deviation may be due to light scattering from the probe laser. The best-fit values of k,, kz and the initial branching ratio A within the pressure range studied (Pall = 0.11-1.03 Torr) are listed in Table 1. The second-order reaction tends to dominate with an increase in &,,rz.

349 TABLE

1

Fitting parameters

of ZnI

temporal

profiles at various

ZnIs pressures

pressure P (Torr)

First-order decay constant k, (104 s-r)

Second-order decay constant k, (arbitrary units)

WZnGd~~

1.03 0.88 0.76 0.65 0.51 0.37 0.24 0.11

1.50 1.29 1.23 1.03 0.86 0.63 0.48 0.36

0.96 1.12 1.41 0.86 0.88 0.77 0.69 0.44

1.78 1.96 1.57 1.53 1.37 1.42 0.99 0.36

=I*

Initial branching

ratio

The apparent first-order decay constant kl includes some diffusional loss of ZnI to the cell wall. The effect of diffusional loss can be estimated with some appropriate approximations; the apparent first-order decay constant kl is written as [lOI

kl=BP-+-kRP

(6)

where kR is the second-order reaction constant (Torr-’ s-l) for the reaction of ZnI with ZnIz, P is the pressure of ZnIz and B is a quantity which represents the contribution of the diffusional removal. B can be denoted as [ll]

where r and 2 are the radius and length of the cylindrical cell respectively, D, is the diffusion coefficient at the standard state (1 atm, 0 “C), T, and P, are the standard temperature and pressure respectively and the quantity jo, 0 = 2.40483 is the first zero point of the Bessel function (J,,(j0,0) =O). Equation (6) can be rewritten as k,P=B+kRPZ

if3

We have plotted klP against P2 in Fig. 5. The points for the lower pressure region (P”
HgBr2 + Br

In the case of Cd&. Kasatani et al. [9] observed a second-order and indicated the possible process for the decay as follows Cd1+Cd1-

Cd1,+Cd

or Cd1+1-

Cd + 12

decay of Cd1 radicals,

350 1.5-

I

I

1

I

I

I

I

I

1

1

I

I

I

I

I

1

I

0.5

P2/ Torr2

I 0

I

1.0

Fig. 5. Dependence of klP (k,, apparent first-order decay constant; P, pressure of ZnI,) of ZnI on P2, as determined by the (0, 0) transition at 500 “C. The slope shows the second-order rate constant for the reaction of ZnI with ZnIs-

In the present case, the decay of ZnI radicals is due to a combination of firstand second-order decays. The probable processes for the second-order decay are similar to the CdIz case ZnI + ZnI +

ZnIz + Zn

or ZnI+I-

Zn+I,

For the first-order decay, the reaction ZnI + ZnIz -

ZnIz + Zn + I

is unlikely, since it involves either endothermic collisional dissociation of a single ZnI molecule or the simultaneous rupture of two Zn-I bonds. The most probable reaction will be similar to the HgBrz case, i.e. znqx

cz) + 12w

Z&+1

In this case the experimental result indicates that PI, is proportional to Pznll, and the rate constant kR must be multiplied by an unknown proportionality constant to obtain the second-order rate constant for the reaction of ZnI with Iz. 3.2. Decay of ground state Zn(42 ‘S) Typical data on the temporal variation in Zn(4s2 IS) density, probed by atomic resonance absorption spectroscopy at Th=48Cl “C, are shown in Fig. 6. The temporal variation in the zinc atom density obeys fist-order kinetics. The decay constants of the zinc atoms at various ZnIz pressures are listed in Table 2. The observed decay seems to be a combination of collisional and diffusional removal. As in the discussion of the first-order decay of ZuI radicals, we have analysed the data using eqn. (8). The result is shown in Fig. 7. The kR value obtained for the Zn +Zn12 reaction is 60.4& 1.9 Torr-’ s-l ((4.70 f0.15) X lo-” cm3 molecule-’ s-l) and B=113 f 9 Torr S-l*

351

G2’ s

--.

d

zL

I

I

‘-,

.Z

-.-..j;.< _“,:_._ ‘._-‘I.;.>., -lb

l-

--if&Y C 1 OO Time

-._ ,~_.Y._ .. .I,.. ;:_L '.

I 40

I 20 after

dissociation

pulse

/ms

Fig. 6. Temporal variation in zinc atom density at 480 “C, Pznlz=2.29 generated from the photodissociation of ZnI, at 266 nm. TABLE

Torr.

Zinc atoms are

.2

First-order

decay constants of ground

state zinc atoms at various ZnIz pressures

ZnI, pressure P (Torr)

First-order decay constant of Zn ki (loz s-l)

3.16 2.29 1.80 1.45 1.28 1.20 0.90 0.63

2.3 1.9 1.7 2.1 1.7 1.6 2.0 2.2

0'

0

I

I

2

4

6

8

10

P2 I Torr2 Fig. 7. Dependence of k,P (kl, apparent first-order decay constant; P, pressure of ZnIz) of zinc atom on Pz at 480 “C. The slope shows the second-order rate constant for the reaction of zinc atoms with ZnI,.

Three

decay

modes

Zn(4s2 ‘S) + Zn12 $

$y+

of Zn(4s*

‘S)

atoms must be considered

I2

\ (ZnI), Of these three, the formation of ZnI radicals can be eliminated because it is an endothermic reaction. The formation of the zinc dimer is also excluded because Znz is reported to be very weakly bonded (0.056 ev) [12]. The last process is the most

352 I

I

1

I

XeCl

I

0

350

I

400

I

I

450 500 hobs /nm

I

550

600

Fig. 8. Emission spectrum of excited zinc atoms formed during focused irradiation by an XeCl laser. Assignments of lines are shown.

plausible. Although ZnI dimers have not been reported, the formation of alkali halide dimers has been observed [13]. The most reasonable decay process for Zn(4s2 ‘S) atoms which diffuse out of the viewing zone is Zn(4s2 IS) + I2 + Wall +

ZnI, + Wall

From the experimental value of B and expression (7), .the standard counterdiffision coefficient (Ds) is obtained as (7.2 f 0.6) x 10e3 cm2 s-r. D, for the zinc system in Zn12 is not known. We estimated it from the kinetic theory of gases, assuming that the collision cross-sections of zinc and ZnIz are approximately 1 nm’. Then, D, can be estimated to be approximately 1.4~ low2 cm2 s-l, which is consistent with the experimental result. 3.3. Formation of excited zinc atoms The emission spectrum due to excited zinc atoms, which are formed during focused irradiation with the XeCI excimer laser, is shown in Fig. 8. The line intensities from 5 3S1 and 4 ‘Dz levels show a quadratic dependence on the laser power. By reference to the excitation energy diagram (Fig. 2) it can be seen that three or four photons are necessary to induce these atomic emission lines. Zn* atoms can be generated by either of the following mechanisms: (a) ZnI(X), produced by one-photon dissociation, absorbs a further two or three photons to give Zn*; (b) Zn12 directly absorbs three or four photons to give Zn*. The emission from excited ZnI(C+ X) was hardly detectable for the experimental conditions in which Zn* emission was observed. This makes case (a) less probable. Many atomic emission lines caused by multiphoton absorption processes have also been observed in the photodissociation of Cd& [9], showing that a variety of energetically feasible excitation processes occur with little selectivity in the formation of cd *. A similar situation was found in the present study for the formation of Zn* from Zn12.

Acknowledgments The authors are grateful to Messrs. I. Nakano and Y. Tanaka Their thanks are also extended to the referee for valuable comments.

for assistance. This work was

353

partly supported by a Grant-in-Aid for Scientific Research (59470010) from the Ministry of Education, Science and Culture of Japan.

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