Collisional quenching of electronically excited zinc atoms

Collisional Quenching of Electronically Excited Zinc

The Journal of Physical Chemistry, Vol. 83, No. 9, 1979

1145

Collisional Quenching of Electronically Excited Zinc Atoms. Cross Sections and Exit Channels W. HI. Breckenridge" and Anlta M. Renlundt Depattment of Chemistry, University of Utah, Salt Lake City, Utah 84 112 (Received December 4, 1978) Publication costs assisted by the Petroleum Research Fund

Absolute cross sections have been measured for the quenching of excited Zn('P1) and by several simple molecules. The magnitudes of the cross sections are similar to those determined previously for the analogous Cd(l:P1)and Cd(3P~) states. For Zn('P1), the cross sections are all large and correlate with the long-range forces c6 parameter to the power 0.9 f 0.1. Major exit channels for Zn(lP1) vs. Cd('P1) are dramatically different for several quencher molecules, however. Spin-forbidden collisional production of the 3PJstates, for which branching ratios approach unity in the quenching of Cd('P1) by N2and the alkane hydrocarbons, was not detected in the Zn('P1) case for any quencher molecule. Also, metal monohydride production in the quenching by CHI appears to be a major exit channel for Zn('P1) but is negligible for the quenching of Cd('P1) by alkane hydrocarbons. Quenching of Z~("J) occurs with high cross section for the isotopic hydrogens but with only moderate-to-low cross sections for N2, CO, and CH4,similar to earlier results for Cd(3PJ)and Hg(3P1). Possible mechanisms for the quenching of the lP1 and 3 P states ~ of Hg, Cd, Zn, and Mg are discussed, such as charge-transfer surface crossings and adiabatic or nonadiabatic chemical reaction pathways via H-M-H or H-M-R potential surfaces (where M is the metal atom and R an alkyl radical).

Introduction

There is growing interest in the rates and mechanisms by which excited electronic states of atoms lose their energy in simple bimolecular ~ollisions.l-~On the one hand, advances in technology and theory have stimulated interest in the determination of "state-to-state" rates of electronic energy dissipation as indicators of the interaction potentials and dynamical restraints which characterize elementary collisional deactivation. On the other, modeling of known and hypothetical laser systems often requires detailed information about the quenching of excited atomic states as well as the major exit channels by which the electronic energy is dissipated. We have been interested for some time in characterizing the rates and modes of collisional energy dissipation in the quenching of the first excited singlet ('Pl) and triplet (3PJ)states of group 2 and 2a metals (Le., such as, Hg, Cd, Zn, Mg).6-15 Many of the conclusions of earlier studies of Cd('P,) and Cd(3PJ) quenching using the technique of resonance-radiation flash photolysislOJ1have recently been confirmed6yg using direct laser excitation and detection, as well as time-resolved fluorescence. One of the interesting aspects of studying collisional quenching of the excited states of the Hg, Cd, Zn, Mg series is that the spin-orbit coupling varies from 2000 to 20 cm-l. Thus, one might expect differences in dominant exit channels in the quenching of the excited states of these metal atoms as well as possible variations in overall quenching rates with metal and/or quencher gas. For instance, the process N

-

-

M('P,) + Q M ( 3 P ~+ ) Q (1) has been shown"J6 ti) occur readily for N2,CO, and alkane hydrocarbons when M = Cd or Hg. In fact, recent laser measurements in our laboratories indicate that when M = Cd, the branching ratios for Cd(3PJ)formation when Q = N2, CHI, or CJ&, are nearly unity, and that net quenching takes place a t essentially every collision.6 Qualitative results of other worker@ for M = Hg indicate

* Dreyfus Foundation Teacher-Scholar,

1973-1978. Department of Physical Chemistry, Cambridge University, Lensfield Road, Cambridge, England.

0022-3654/79/2083-1145$01 .OO/O

similar behavior for the deactivation of Hg('P1). It is interesting to determine whether such remarkably efficient spin-forbidden processes persist to states such as Zn('P1) and Mg(lP1), where the spin-orbit interaction is progressively smaller. Deactivation of excited 0('D2) by N2 is certainly a very efficient process,17 for example, even though spin-orbit coupling in oxygen is small, apparently'8J9 because the ground-state singlet N20 complex initially formed has a deep potential well and thus a long lifetime, allowing eventual singlet-triplet surface crossing before dissociation back into the entrance channel. We report here some results on quenching cross sections and exit channels for the quenching of Zn(lP1) and Zn(3PJ) by several simple molecules, using the technique of resonance-radiation flash photolysis. Unfortunately, it is not possible to excite Zn('P1) at 2138 A with our present dye-laser apparatus6s9because the potassium pentaborate frequency-doubling crystal cannot produce radiation below 2170 A. Since the resonance-radiation flash photolysis measurements for excited Cd using the same flash lamp and reaction vesselll have been confirmed by direct laser measurements! however, there is every reason to believe that the measurements reported here for Zn excited states are also reliable. Although it was not possible to obtain the comprehensive information gathered in the Cd('P1) and Cd(3PJ) studies reported earlier,8J0J1J5in part due to the higher temperatures necessary and the more reactive character of Zn vapor (as well as the premature demise of the expensive multielectrode flash lamp because of vitrification), sufficiently interesting differences between the quenching of Zn vs. Cd and Hg excited states were observed to prompt publication of the results, Experimental Section

The apparatus and general technique have been described previously.'lJ5 Briefly, high concentrations of excited metal atoms are generated in a metal vapor/buffer gas mixture by an intense 50-fis pulse of resonance radiation of the metal of interest, generated by a specially constructed multi-electrode flash lamp. For the present experiments, the reaction vessel saturator and the lamp contained Zn turnings. Intense emission at 2138 A (lP1 lS0)and 3076 A (3P1 'So) was emitted by the flash

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0 1979 American Chemlcal Society


1148

W. H. Breckenridge and A. M. Renlund

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lamp when the temperature of the system was raised to 350 "C. Conventional kinetic absorption spectroscopy, using a Kr-filled flash lamp, was used to detect any absorbing species resulting from exposure of the reaction vessel to the zinc resonance-radiation excitation flash. It was also possible to monitor Zn(lP1) and produced in the reaction vessel by utilizing fluorescence at 2138 and 3076 A, respectively. Details of the quantitative collection of both absorption and fluorescence data using the apparatus have been reported previously1'J5 and need not be repeated here. Up to ten flashes were required to obtain sufficient fluorescence for accurate measurements in the Zn system. The product species ZnH and ZnD were detected by means of the C-X(0,O) band at 2426 A, and the absorptions were shown to follow Beer's law by using a variable path method described elsewhere15 (OD = a. (bc)",a = 0.95 f 0.10, where OD is optical density at the band peak, b is the path length, and c the concentration of ZnH or ZnD). Sources and purification procedures for the gases used in this study have been described p r e v i ~ u s l y . ~ ' J ~

Results For the experimental temperature conditions of 352 "C, the 2138-A lamp emission is absorbed strongly by Zn vapor plus 300 torr of helium, producing sufficient Zn(lP,) for quantitative quenching measurements using the resultant Zn(lP1 lS0)fluorescence at 2138 A emanating from the reaction vessel. All experiments were performed at a total pressure made up to 300 torr with inert helium buffer gas in order to minimize any changes in absorption line broadening, and thus total light absorbed, when a quencher gas is added, and to prevent any temperature rise. The presence of very small amounts of Zn(3Po,1,2) states could also be detected by kinetic absorption spectroscopy as well as by very weak Zd3P1 'So) fluorescence at 3076 A. The small concentrations of Zr$PJ) states apparently result from weak absorption of lamp 3076-A radiation by means of the spin-forbidden Zn(1S0-3P1) transition, followed by an equilibration of the J = 0, 1, 2 levels which is fast compared to the observation time. (The Zn(3P2-3Pl) splitting is only 1.1 kcal/mol, and kT is 1.3 kcal/mol at 352 "C.) Quenching Cross Sections for Zn(lP1). The overall quenching rates of Zn('P1) by several gases at 352 "C were determined by measuring the diminution of the 2138-A fluorescence intensity when concentrations of each quencher gas were increased. Because the effective radiative lifetime of Zn(lP1), even under the moderate radiation imprisonment conditions in the reaction vessel, is s, while the duration of the on the order of only 2 X excitation pulse is -500 X lo-' s, the fluorescence intensity for an one experiment will be exactly proportional to the 2138- excitation intensity, El. The following equations for steady-state Stern-Volmer fluorescence quenching are therefore valid: Zn(lSo) + hv (2138 A) Zn(lP1) El Zn(lP1) Zn('So) + hv (2138 A) k,l

I 00

10

05

K

-+ -

-

Zn(lP1) Q quenching kQ I_ O -- 1 + (1) I kr1 where I is the fluorescence intensity at 2138 A with a concentration [Q] of a quencher gas present; Io is the fluorescence intensity when [Q] = 0; k,, is the effective rate constant for the escape of imprisoned 2138-A fluorescent photons from the reaction vessel (different from the

20

25

Flgure 1. Representative Stern-Volrner plots of the relative ernlssion intensity at 2138 A vs. pressure of added quencher, Q.

TABLE I: Relative Rates of Quenching of Zn( 'P,) by Several Gases quencher

kQ/kH,a

2.12 2.24 1.71 1.12 1.14 1.00

-+

-

I5

101 (torr)

0.88

N2

0.79 0.51

T = 352 "C.

limiting rate of natural radiative decay at very low Zn vapor concentrations); and kQ is the total rate constant for the removal of Zn('P1) states by quencher Q (Le., any collisional process which does not produce a photon at 2138 A). Quenching by helium gas is assumed to be negligible, as was shown to be the case for Cd('P1) under similar experimental conditions. The main difficulty in establishing absolute k , values for various quenchers is that k,, must be calculated using a theory of radiation imprisonment. However, relative values of kQ may be determined with no difficulty, since there is theoretical and experimental evidence that k,, will remain constant (for a fixed detection geometry) with increasing pressures of a quenching gas at steady state even under high-opacity radiation imprisonment conditions.20i21 Data for quenching of 2138-A fluorescence for a range of gases was shown to fit the Stern-Volmer expression, and representative plots of Zo/Z vs. [Q] are shown in Figure 1. Relative values of kQ determined from data such as this are shown in Table I. Slopes of the Stern-Volmer plots were determined by a least-squares computer routine. The fluorescent intensity at 2138 A, I, could be reduced to zero by the addition of sufficient quenching gas, showing that the scattered light reaching the spectrograph from the flash lamp was negligible compared to the fluorescence from the reaction vessel. Conversion of the relative quenching rates of Table I to absolute quenching cross sections requires calculations of k,, the effective rate constant for radiative escape of resonant 2138-A photons from the cylindrical reaction vessel, using a theory of radiation imprisonment (that of Holstein,22as modified by WalshZ3). The validity of the calculations, as well as the assumption that addition of quencher gas does not change the effective photon escape rate, have been discussed previously'l for the similar case

The Journal of Physical Chemistty, Vol. 83, No. 9, 1979


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TABLE 11: Cross Sectionsa for Quenching of Zn('P,) and Cd('P,) by Several Gases UQ'

quencher i-C,H,,

CJ[*

co

N* DZ HD H2

Cd( 'P,)" 131 110 38 47 35 31 (-28)d 26

a U Q = k&; is= [ 8 h T / n p ] 1 ' 2the , mean Boltzmann T = 625 K. T = 390 K. Values taken from speed. Relative value determined in ref ref 6 unless indicated. 10 assumed correct.

of Cd(lP1) fluorescence at 2288 A under very similar conditions off opacity, geometry, etc., and need not be repeated. Direct, time-resolved experiments6 using laser excitation have since yielded absolute Cd(lP1) quenching cross sections which are within about 20% of the values calculated previously using imprisonment theory,ll so that use of the imprisonment theory in the Zn(lP1) case is also unlikely to result in major error. Shown in Table I1 are the absolute quenching cross sections calculated in this manner for Zn(lP1), along with the corresponding cross sections for Cd('P1) quenching, for comparison. Exit Channels i n the Quenching of Zn('P,). (i) Production of In marked contrast to the efficient collisional irltersystem crossing of Cd(lP1) to the C d ( 3 P ~ ) states which has been observed for several molecular quenchers,6111the results of the experiments reported here have provided no evidence for Zn(3PJ ) production in the quenching of Zn('PI) by any quenching molecule studied. Only a steady decrease in concentrations with added quenehers was found in both absorption and emission experiments. In fact, as little as 0.1 torr of Hz, HD, or Dzquenched ZII(~PJ)below the detection limits. It should be noted that because of the approximate tenfold longer radiative lifetime of Zd3P1)compared to Cd(3P1), fluorescence detection of ZII(~P)is much less sensitive than for Cd(3P),if there is any deactivation of the 3PJstates, but this limitation should not apply to ZII(~P,)absorption measurements during the excitation flash. In fact, in the absence of collisional quenching of Z I I ( ~ P ~the ) , concentration of any Zn(3PJ ) formed by deactivation would be higher than that of Cd(3PJ) during the excitation flash because of the longer effective radiative decay constant for ZII(~P~). The 3P 3Dtransitions by which the 3P atoms are detected in absorption should have comparable oscillator strengths for Cd and Zn.24 For Nz, CHI, and CO it was possible to observe the decrease in Z~I(~PJ) concentration with added pressures of the quenclhers. The processes involving a quencher Q (in addition to those listed above) which lead to possible production and quenching of Zn(3PJ) are summarized as follows:

-

Zn('P1)

+Q

+

quenching not producing Zn('PI)

+Q

-

-

kQ - k d

Zn(3PJ)

Zn(lS0) -k hv (3076 A)

+Q

ZII(~PJ

--, Zn(lSo) + hu (3076 A) Zn(3PJ) + Q

-

,x /

A'

Zn(lP, ) b 106 99 68 34 30 19 19 13 12

CZHL CH,,

/Q=CO

1147

quenching

kd

E3 kY3

k,

I 0 00

0 40

I20

0 80

I60

[Q] (torr) Figure 2. Stern-Volmer-like plots of the relative emission intensity at 3076 A vs. pressure of added quencher, Q.

It is assumed that all Zn(3PJ) states, whether formed from direct excitation of ZII(~P,)or from deactivation of Zn(lP1), are rapidly equilibrated under the experimental condit i o n ~ .When ~ ~ the relative concentrations of ZII(~P J ) are measured with and without a given concentration of the quencher, [ Q ] ,and the emission intensity of the Zn(3P1-1S0)3076-A resonance line is assumed to be proportional to the total ZII(~PJ) concentration, the following expression can be derived: I(3076 A),=, -

where I(3076 A),=, and I(3076 A), are the integrated fluorescence intensities per flash of 3076-A radiation with no quencher present and with a quencher concentration of [Q]present, respectively; and E l and E3 are the rates of direct excitation by the flash lamp of Zn(lP1) and ZII(~P,), respectively. If ZII(~P,)results entirely from direct excitation by absorption of 3076-A radiation from the flash lamp (i.e., k d = O), then the portion of the equation in large brackets becomes unity and a normal Stern-Volmer-like equation for quenching of 3076-A resonance radiation results. Shown in Figure 2 are plots of I(3076 A) ,0/1(3076 A), vs. [Q]. However, linearity of I(3076 A)~,o/?(3076A), with [Q]could also result if k , [ Q ] / k I 3>> 1 and hdE1[Q]/ +,E3 >> [ k ~ [ Q ] / h+, ,1ouer the range of [ Q ]inuestigated. Since E1/E3 can be estimated to be on the order of 50-100 from the known E,/E3 ratio in the analogous Cd system where kd/kQhas now been determined for Nz,6the data in Figure 2 can be used t o show that the above inequalities can only be satisfied if kd/kq approaches unity and kg= hd (i.e., if for all three gases Zn( P1)is deactivated to Zn( PJ) at rates approaching gas-kinetic collision rates, and Z ~ I (J ~ ) is P also deactivated at similarly high rates by all three gases). This is extremely unlikely, since the quenching rate of Zn(3PJ) by CH4 has been determinedz6 to be approximately two orders of magnitude slower than gas kinetic, and the net quenching rates of the analogous Hg(3P~), Cd(3PJ), and Mg(3P~) states by N2are all slower than gas kinetic rates by at least three orders of magnitude.14J5 It is also virtually impossible that, for all three of these gases, the extrapolated value of I(3076 A), as [Q] 0 just happens to coincide with I(3076 the

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amount of 3076-A fluorescence with only He gas present. (See Figure 2.) We therefore conclude that at least for CO, N2,and CH4,production of ZII(~P~) is indeed a minor exit channel for the quenching of Zn(lP1). That being the case, the data in Figure 2 merely represent Stern-Volmer-like quenching plots for weak direct excitation of Z~I(~P,). In Table I11 are the cross sections for quenching of equilibrated ZII(~P~) by Nz, CO, and CH4 calculated from these plots. It can be shown that radiation imprisonment of 3076-A resonance fluorescence is completely negligible under the experimental conditi~ns.~’ Cross sections for quenching of Hg(3P,,l), Cd(3PJ), and Mg(3PJ) by the same gases are included in the table for comparison. We prefer to quote the cross section for quenching of ZII(~PJ)by CH4 as a formal upper limit, since it was shown that the 3076-A intensity when CHI was present decreased steadily with number of flashes when a larger number of excitation flashes were used than for the Stern-Volmer measurements. This was probably due to accumulation of Hz and/or alkene products in the system from efficient chemical exit channels in the simultaneous quenching of the much larger amounts of Zn(’P1) by CH4. From the efficient quenching observed, the cross sections for quenching of ZII(~P,)by Hz, HD, and Dz must be larger than 10 A2. (ii) Production of ZnH. The species ZnH (or ZnD) was detected readily in absorption in the quenching of Zn(lP1) HD, Dz, and the alkane hydrocarbons. When ZnH by Hz, was utilized as a long-lived “marker” for the quantitative measurement of Zn(lP1) quenching by CH4 (see ref 10 and 15 for the details of the technique), the quenching rate determined was virtually identical with the value obtained by the 2138-A fluorescence quenching method described above. It is possible that some of the ZnH is due to reaction of CH4 with Z I I ( ~ P ~ )It. was shown above that deactivation of Zn(lP1) to form ZII(~P,)does not occur with CH4 so that only the Zn(3P) formed by direct excitation could be responsible. Comparison of the quenching half-pressures (the pressure of added quencher sufficient to quench half of the excited state concentration) for reaction of CHI with Zn(lP1) and ZII(~P~), 0.73 and 0.27 torr, respectively, indicates that the ZII(~P,)is completely quenched a t pressures of CHI before the ZnH yield is maximized, however. Therefore, most of the ZnH observed must be due to the direct reaction of Zn(’P1) with CH4.

Discussion To our knowledge, there are no previous determinations of Zn(’P1) quenching cross sections with which to compare the values reported here. It is interesting to note, however, from Table I1 that the cross sections for Zn(lP,) quenching closely parallel those for Cd(lP1) quenching, particularly those for the hydrocarbons. The cross sections for Cd(’P1) quenching have been shown6 to correlate with the longrange forces c6 parameter to the power 0.66 f 0.10. In Figure 3 we present a log-log plot of Zn(’P,) quenching cross sections vs. the c6 parameter for the Zn(lP1)quencher interaction (estimated using procedures described previously).ll There is obviously a good correlation, and the slope of the least-squares line through the data is 0.88 f 0.11 in this case. The different slopes for the Cd(lP1) and Zn(lP1) may not be significant, considering the limited number of quenchers studied, but it appears safe to assert that the cross sections do correlate with the c 6 parameter to a power between 0.5 and 1.0. (Interestingly, if only the cross sections for quenching of Cd(’P1) and Zn(’P1) by the simple alkane hydrocarbons are considered, the slope of the log-log plot is 1.10 0.08.) Setser

*

W. H. Breckenridge and A. M. Renlund

log

c6

Figure 3. A log-log plot of Zn(’P,) quenching cross sections vs. the C6long-range parameter. See text.

and co-workers have demonstrated that the larger cross sections for quenching of the excited metastable states of inert gas atoms correlate approximately linearly with the c 6 parameter, whereas a somewhat limited amount of data for the larger cross sections for quenching of electronically excited alkali atoms appear to correlate with the c6 parameter to a lower power, -0.4.11128 Such correlations have been discussed1vz8in terms of mechanisms whereby quenching occurs at every “collision”, Le., an encounter in which excited atom and quencher molecules approach within some critical distance under the constraints of a balance between the long-range C6/r6 attractive potential and the repulsive centrifugal potential which arises from the necessity of conserving overall angular momentum. Measurements of the velocity dependence of quenching processes involving excited inert-gas states as well as excited alkali states, which provide a more sensitive test of mechanism, also appear to be c o n s i ~ t e n t ~ with ~ - ~ l such models. These “orbiting” or “absorbing sphere” models assume that there is an efficient mechanism coupling the perturbed excited atom state into available exit channels once the “capture” (entrancechannel) portion of the collision has occurred. This is certainly consistent with the observations that a wide variety of exit channels are observed for several molecular quenchers of the same excited atomic state, and that a different exit channel may predominate when two different excited atomic states are quenched by the same molecule. For example, the major exit channel for quenching of Cd(lP1) by CH4 is formation of Cd(3PJ), but Zn(3PJ) production is at best a minor exit channel in the quenching of Zn(lP1) by CH4 even though the total quenching cross sections are virtually identical. Most of the quenching cross sections for the 3P states of Hg, Cd, Zn, and Mg do not correlate at all with the c6 parameter (see Table 111, for example), and some of the cross sections are up to four orders of magnitude smaller than would be predicted on the basis of “entrance-channel” control of the quenching process (i.e., quenching at every “collision”). The implication is that the efficient coupling mechanism(s) available to the ‘P1 states are often not available to the lower-energy 3Pstates (although in certain cases other mechanisms may become more efficient, such as the triplet-triplet exchange interaction known to be strong for quenchers with low-lying triplet states like the alkene hydrocarbons). We have discussed in an earlier paper1’ the hypothesis that for some quenchers the accessibility of a charge-transfer M+Q- surface may be a requirement for efficient quenching, via curve crossing, so



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TABLE 111: Cross Sectionsavb for Quenching of the 'PJ States of Hg, Cd, Zn, and Mg by N,, CO, and CH4

excitation excited energy, state kcal/mol Hg('P,) 11:2.7 Hg('P,) 10'7.6 Z ~ ( ' P J ) 9'2.4,92.9, 94.0 Cd('Pj) 86.1, 87.6 Mg('Pj) 62.4, 62.6, 62.6

T, K 298 298 625, 578

effective cross section measured .('P,)" .('Po) 0 . 5 3 ~ ( ~ P+, )l.OOo('P,) + 0.650(~P,) I . ~ ~ ~ ( J+P1 ,. o) o o ( 3 ~ , ) O.l20('PO)+ O.34u('P1)+ 0.54~(~P,)

I

553 800

quenching gas

co

N,

Collisional quenching of electronically excited zinc atoms

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