Cesium halide photodissociation cross sections

1. Quant. Sprctrosc. Radial.

Transfer, Vol. 17, pp. 73-76. Pergamon Press 1977. Printed in Great Britain

CESIUM HALIDE PHOTODISSOCIATION CROSS SECTIONS A. MANDL Avco EverettResearchLaboratory,Inc., 2385RevereBeachParkway,Everett,MA 02149,U.S.A. (Received11March 1976) Abstract-The cross sections for the photodissociationof CsCI,CsBr and CsI have been measuredusing a cesium halide seeded shock tube. These measurementswere made by using the ratio of two absorption signals: one in the incident and one in the reflected shock region. The cesium halide was the absorption species in the incident shock region and a halogen negative ion species absorbedin the reflectedshock region.Previouslymeasuredand calculatedhalogennegativeion photodetachmentcross sections were used in calculatingthe cesium halidephotodissociationcross section. Measurementswere madebetween4000and 2000A at 1900°K.

INTRODUCTION

As A CONTINUATION of our high temperature CsF photodissociation cross section work,“’ we have measured the remaining cesium halide photo-dissociation cross sections

csx + hv +

cs(2s) + X(‘P),

(1)

where X = Cl, Br and I. These measurements were performed at 1900°K. The only other measurements of these cross sections have been made at T -800°K.‘Z’ The temperature dependence of these cross sections is important in establishing the shape of the molecular potential curves. A nice feature of the measurement at high temperature is that it reduces the uncertainty in the density measurement due to polymer formation.‘“’ The way that both incident and reflected shock tube measurements were used to relate the cesium halide density to the halogen negative ion density was described in a previous publication,“’ where the CsF density [CsF] was related to [F-l. Essentially, the [X-l density is measured in the reflected shock using the photo-absorption cross sections for the various X- which have recently been measured.“) One can use the shock equations to calculate the density change across the reflected shock and combining this with the fact that alP the CsX dissociates into X- and Cs’ one can relate [X-l in the reflected shock region to a [CsX] in the incident shock region. As is shown below one can thus determine the photodissociation cross sections of the various cesium halides. EXPERIMENTAL

The measurements were performed in the same apparatus in which the CsF photodissociation cross section work was carried out. This is a shock tube in which an N2 background test gas can be seeded with the various cesium halides; a detailed description appears earlier.‘6’ Measurements are made behind incident and reflected shocks. In order to reduce effects of boundary layers a 3.5 in. square cross section box with knife edges was positioned in the center of the 6in. shock tube in the diagnostic section. The cesium halides (CsX) are introduced by vaporization of the relevant salt into the background test gas followed by self-nucleation forming small particles. Typical particle sizes have been shown to be -0.05 &@for CsF. Since all the cesium halides have similar chemical properties (e.g. melting points are within 63°K and boiling points are within 50°K of each other), the same technique was used in the present work. Typical mole fractions of CsX in the shock test gas were from 0.01 to 0.1% and typical initial N2 pressures are 4 torr. The N2 density increases by about a factor of 6 in the incident shock region and about a factor of 25 in the reflected shock region, relative to the initial pressure. The mixture (N2 and CSX,~,-& is subjected to incident shock heating, raising the temperature, (T2) to - 1900°K.This temperature will ablate the CsX (forming CsX vapor) but not dissociate it. This reaction is written as 13

A. MANDL

74

The gaseous mixture is subsequently subjected to reflected shock heating, which essentially doubles the temperature, causing the CsX to dissociate by the following reactions: CSX+N~+CS’+X-+N~,

(3)

CsX+Nz+Cs+X+N1.

(4)

It has been shown by BERRYet uZ.‘~.”that reaction (3) is overwhelmingly favored for all the cesium halides. Thus, large densities of negative haIogen ions are produced behind the reflected shock. Absorption measurements are made in both the incident and reflected shock as described previously.“’ These measurements were made between 4000 and 2000 A. The absorption in the incident shock is due to CsX photodissociation [eqn (l)] while the absorption in the reflected shock is due to X- photodetachment, viz. X-(‘S) + hv -+X(*P~,~.,,~) + e-.

(5)

The cross sections for photodetachment of the halogen negative ions have been calculated”’ and measured.‘4*e1’)Since our measurements of the photodetachment cross section [cr(X-)] are in good agreement with the theory, we have used the theoretical values of u(X) in determining the absolute values of the CsX cross sections. Thus, by noting that all the CsX dissociates into ions, one can use [CSXI = p*/p5w10.

(6)

The density ratio between the incident shock, pz, and the reflected shock, p5, is easily calculated and [X-l,, is the initial density of X- immediately after the passage of the reflected shock and completion of reaction (3). The theoretical cross section for photodetachment is used to give lx-l

=

o

ML IL) u(X_)I ’

where Z, is the incident light, IOis the transmitted light just after reaction (3) and 1 is the optical path through the shock tube. Similarly, the cross section for photodissociation of the CsX is given by

u(CsX)=~, S

(8)

where the primes refer to measurements in the incident shock region. Thus, combining eqns (6)-(S), one obtains u(CsX) = (p5/pMX-)

#.

(9) I

0

For each shock tube run the terms on the right hand side of eqn (9) are evaluated. Thus by varying the wavelength of measurement between 4000 and 2000 A for each cesium halide salt the spectral dependence of the photodissociation cross section of all the cesium halides has been mapped out. The experimental procedure used was described in Ref. (1). RESULTS The measured cross sections for photodissociation of CsCl, CsBr and CsI are given in Figs. l-2, respectively. The solid circles are the results of the present experiments. Each point

Cesiumhalidephotodissociationcrdss sections -1s XI0

I

24-

-

I

I

I

I

I

I

DAVIDOVITS 8 BRODHEAD T-B~o~K 0 PRESENT EXPERIMENT T- 1900°K

2O-

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-

2

12z

-

b”

84-

0

A

h

3200

3000

2800

2600

2400

2200

PHOTON WAVELENGTH

EM04

Fig.

2000

(i,

1. The measuredphotodissociationcross sectionof CsCI.

-18 xl0

I -

I

I

I

B BRODHEAD

l PRESENT

“g

I

-DAVlDOVITS

T-

EXPERIMENT

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20 -

IO -

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I 3600

3400

3200

3000

2800

PHOTON

E6603

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2400

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2200

2000

(b,

Fig. 2. The measuredphotodissociationcross sectionof CsBr. -16 XI0 ,

I

I20

-

I

DAVIDOVITS 0

PRESENT

I

I

I

e BROADHEAD DATA

T-

I

T-

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I

I

725OK

1900°K l

IOOl “E u

BO* .,

’

60-

4000 EM06

3600

3600

3400

3200

PHOTON

3000

2600

WAVELENGTH

2600

2400

(A,

Fig. 3. The measuredphotodissociationcross sectionof CsI.

2200

2000

16

A.

MANDL

represents the average of several runs. All measurements were made at T = l!NO“Kf 100°K. There are no other measurements of this cross section at these high temperatures; we have, however, plotted for comparison the measured cross sections of DAVKIOVJTS and BRODHE,@ as solid lines. Their measurements were all made at substantially lower temperatures, as noted on the figures. The general features of the cross section remain about the same in the two works. The longest wavelength peak, however, is smaller at higher temperature for both CsCl and CsBr while it remains about the same for CsI. A good discussion of the general shape of the potential energy curves for the alkali halides is given in Ref. (12). From that discussion it is clear that one cannot predict how the cross section will change with temperature unless one knows the shape of the lower potential energy curve, the upper potential energy curve (which can be slightly attractive) and the overlap of the two curves. A comparison of the present experimentally measured “first” (or long wavelength) peak cross section with those of Brodhead and Davidovits is shown in Fig. 4. The position of the maxima for both cases is the same and one notices a decrease in the value of the peak cross section as the wavelength of the first peak decreases. Since the two sets of measurements were made at different temperatures, and therefore represent different vibrational population distributions, one should not be surprised at differences in the size of the maxima of the various peaks.

Fii. 4. Comparisonof the measuredpeak values of the photodissociationcross sections of the stablecesium halides.The filled-insymbolsare the lower temperaturemeasurementsof Davidovitsand Brodheadwhereas the open symbols are from the present experiment.Our previously measured”’value for CsF has been includedfor completeness. Acknowledgement-The authorwishes to gratefullyacknowledgethe technicalassistancereceivedfromC. D~RAKJ~UR~AN in the operationof the shock tube and associated experimentalequipment.

REFERENCES 1. A. MANDL, JQSBT 11, 1197(1971). 2. P. D~vmovrrsand D. C. BRODHEAD, J. Chem. Phys. 46, 2%8 (1%7). 3. M. EISENSTAL~T, G. M. ROTHBERG and P. KUSCH, 1. Ckem. Phys. 29, 797 (1958). 4. A. MANDL, Phys. Rev. A 14, 345 (1976). 5. R. S. Bears, T. CERNOCH, M. COPLAN and J. J. EWING, J. Chem. Phys. 49, 127 (1968). 6. A. MANDL, J. Appl. Phys. 42, 4936 (1971). 7. J. J. Ewmo, R. MIL.V~IN and R. S. BERRY, J. C/rem.Phys. 54, 1752(1971). 8. E. J. ROBINSON and S. GELTMAN, Phys. Rev. 153, 4 (1%7). 9. A. MANDL, Phys. Rev. A3, 251 (1971). 10. A. MANDL and H. A. HYMAN, Phys. Rev. Lett. 31,417 (1973). 11. G. MUCH and H.-P. POPP,Z. Naturforsch 23a, 1213(1%8). 12. R. F. BARROW and A. D. GAUUT, Proc. R. Sot. (London) Az19, 120 (1953).