1474
The Journal of Physical Chemistry, Vol. 82, No. 13, 1978
W. H. Breckenridge and A. M. Renlund
Quenching of Excited Cadmium( 'P,) Atoms by Several Molecules. Cross Sections and Chemical and Physical Exit Channels W. H. Breckenridge*+ and Anita M. Renlundi Department of Cbemistty, University of Utah, Salt Lake City, Utah 847 12 (Received Februaty 10, 1978) Publication costs assisted by the Petroleum Research Fund
Cross sections have been measured for the quenching of Cd(lP,) by a variety of simple molecules, and major chemical and physical exit channels have been identified in several cases. Except for He and Ar, the cross sections are large and do not vary greatly with quenching molecule, in marked contrast to the quenching of lower-energy Cd(3P~).The cross sections can be correlated with the c6 long-range forces parameter to the power 0.8 f 0.1, similar to the linear correlation demonstrated by Setser and co-workers for the 3P2excited states of Ar, Kr, and Xe. For several gases, notably N2,CO, and the alkane hydrocarbons, the production of Cd(3PJ) by collision-induced intersystem crossing is a major exit channel,and the relative yields of Cd(3P~) were measured. Efficient direct production of CdH was observed in the quenching of Cd(lP1) by H2 but not by the alkane hydrocarbons or NH3. However, indirect evidence was obtained for direct alkane C-H bond sensitization. Quenching of Cd(lP,) by SF6results in the production of CdF, but in smaller yield than in the quenching of Cd(3PJ)by SFs. Several possible mechanisms for the quenching of Cd('P1) as well as other excited states of metal atoms are discussed. It is suggested that a simple charge-transfer curve-crossing theory is useful in rationalizing the magnitudes of cross sections for quenching of N P ,CO, C02, NO, and SF6by Cd(lP1),Hg(3Pl), Hg(3Po),and Cd(3PJ). Another mechanism is likely responsible for the efficient quenching of Cd(lP1) by alkane hydrocarbons, however.
Introduction The quenching of electronically excited atoms has long been of interest to chemists and physi~ists.l-~Understanding the controlling factors for the rate and mechanism by which a fixed amount of atomic electronic energy is dissipated in a collision with a quenching molecule is of fundamental importance, particularly when there are several physical and/or chemical exit channels available. The rate and mode of electronic energy dissipation in such processes has also proved crucial in recent years to the kinetic analysis and design of many laser systems. Studies of the quenching of the second excited state of the cadmium atom, Cd(lP1), are potentially interesting because with many molecular quenchers there are several exit channels, including collision-induced intersystem crossing to form the lower-lying Cd(3PJ) excited level; direct sensitized bond cleavage of the quenching molecule; chemical reaction to form CdX, where X is an atom originally bonded in the quenching molecule; and energy transfer to form ground-state Cd(lSo)and the vibrationally (and/or elecronically) excited quenching molecule. Analogous processes are also possible for the IP1states of the valence isoelectronic atoms Hg, Zn, and Mg, facilitating future comparison. Extensive studies of the quenching of the Cd(3PJ) state have already been performed in our lab~ratories,~-'so that interesting comparisons between rates and modes of energy dissipation of the IPl vs. the 3PJlevels can also be made. Preliminary results had shown that the rates of quenching of Cd('P1) varied much less with quenching species than those of Cd(3PJ).8 Reported here are determinations of the rates of quenching of Cd(lP1) by several gases, as well as identification of major reactive and/or inelastic exit channels for some of the collisional quenching processes. A detailed comparison of the rates and mechanisms of quenching of Camille and Henry Dreyfus Foundation Teacher-Scholar, 1973-1978.
Department of Physical Chemistry, Cambridge University, Cambridge, England. 0022-3654/78/2082-1474$01 .OO/O
Cd('P1) and Cd(3PJ) by the isotopes of molecular hydrogen is presented in the paper following.6
Experimental Section The apparatus and general technique have been described p r e v i ~ u s l y . ~Briefly, high concentrations of electronically excited cadmium atoms are generated in Cd vapor by an intense 40-ps pulse of cadmium resonance radiation from a specially constructed multielectrode flash lamp. The present study was conducted a t 230 "C, where the Cd vapor pressure is sufficiently low that the 3261 A (3P1 'So) radiation from the lamp is not readily absorbed in the 5-mm diameter reaction vessel, and direct production of the Cd(3P1)excited state is barely detectable. It is thus possible, when there is no ultraviolet filtering of the output of the quartz flash lamp, to generate high concentrations of Cd(lP1) only, since intense 2288-A (lP1 lS0)resonance radiation from the flash lamp is absorbed strongly by the cadmium vapor in the reaction vessel. Production of Cd(3Po)or Cd(3Pl)by collision-induced intersystem crossing, or of species such as CdH, CdD, or CdF by direct chemical reaction, can be detected using kinetic absorption spectroscopy with a conventional Krfilled continuum flash lamp. Relative concentrations of Cd(3Po),Cd(3Pl),CdH, and CdD can be determined by plate p h ~ t o m e t r y . ~ Because of a short effective radiative lifetime, the Cd('PI) excited atoms cannot be detected in absorption with the present apparatus. However, it is possible to detect fluorescence a t 2288 8, from the reaction vessel during the lamp excitation pulse and to use the intensity of this fluorescence as a probe of the relative concentration of Cd(lP1) for Stern-Volmer measurements of the quenching rates of several gases. Attempts at photometric monitoring of the 2288-w fluorescence were unsuccessful due to electronic noise from the lamp discharge circuit, so that relative fluorescence intensities were determined using Kodak 103a-0 photographic plates sensitized with sodium salicylate and preexposed to the linear region of plate r e s p o n ~ e . ~Accurate ,~ determination of Stern-Volmer
-
-
0 1978 American
Chemical Society
Quenching of Cd('P,) by Several Molecules
quenching plots required up to six flashes per experiment. The plate-response factor y was determined separately for each plate by measuring the plate optical density of the 2288-A fluorescence intensity (with the reaction vessel filled with pure helium only) as a function of number of flashes. T o check for errors due to reciprocity failure, an experiment was conducted in which a plate y factor was also measured with an amount of N2gas present sufficient to decrease the fluorescence intensity per flash at 2288 A by -35%. The values of y determined for the same plate in the two experiments were 0.600 f 0.007 and 0.595 f 0.012, respectively. Values of y for individual 103a-0 photographic plates used in these experiments varied from 0.50 to 0.69 a t 2288 A. To obtain an indication of the reproducibility of 2288-A pulse intensity, and of this photographic method of measuring fluorescence intensity, a statistical analysis was carried out on data obtained from sets of experiments repeated several times. The standard deviation of the normalized 2288-A fluorescent intensity was found to be only 4.9%, thus establishing the high precision of the experimental method. Sourcespd purification procedures for most of the gases used in this study have been described p r e v i ~ u s l y .The ~ I"CIH1O(Matheson Research Grade) and CF, (Matheson) were freeze-pumped several times before use.
Results Determination of Quenching Rates of Cd('P1). The overall quenching rates of Cd(lP1) by several gases at 230 were determined by measuring the diminution of fluorescence intensity from Cd(lP1)at 2288 A for increasing concentrations of each quencher gas. Because the effective radiative lifetime of Cd(lP1), even under the severe radiation imprisonment conditions in the reaction vessel, is on the order of only s, while the duration of the excitation pulse is -400 X s, the fluorescence intensity for an one experiment will be exactly proportional to the 2288- excitation intensity, so long as the excitation pumping power is not sufficient to effect net depopulation of ground-state Cd(lSo) atoms during the pulse. Experiments have been performed which show that ground-state depopulation occurs to a negligible extent at 230 "C, under the conditions of the quenching experiments reported here.6 The following equations for steady-state SternVolmer fluorescence quenching are, therefore, valid: O C
K
Cd('S,) + hv (2288 A ) Cd('P,) Cd('P,)-+Cd('S,) + hu (2288 A ) Cd(lP,) + Q -+ quenching h~ -+
k,
where I is the fluorescence intensity at 2288 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 escape of 2288-A fluorescent photons from the reaction vessel (different from the limiting rate of natural radiative decay a t very low Cd vapor concentrations); and kQ is the total rate constant for the removal of Cd(lP,) states by quencher Q (i.e., any collisional process which does not produce a photon at 2288 A). Experiments were always conducted with 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 [Q] was varied over a range of a few Torr, and to prevent any appreciable temperature rise during the excitation pulse. Experiments to be discussed below
The Journal of Physical Chemistry, Vol. 82, No. 13, 1978 1475 30
25
20
I.5
IO
0 00
0 40
o eo
I 20
I 60
[Q] (torr) Flgure 1. Stern-Volmer plot of relative emission intensity at 2288 A
vs. pressure of added quencher 0:A, /-C4H10;0,SF,; X, H,; A,NO; 0 , CF4.
TABLE I: Relative Rates of Quenching of Cd(lP,) by Several Gases k~ (relative) quenching gas i-C4Hlo n-C,H10
this work (230°C) 1.50
ref 8 (290 oC)b
ref 20 (280 "C)
ref 22 (95 "C)
5.6 1.10 1.07 1.07 1.00 1.00 1.00 HZ HD 0.91 3.4 0.90 COZ 0.88 1.40 D2 0.74 1.9 "3 3.8 0.69 co N, 0.65 0.60 1.3 CF4 0.14 0.03 Ar (- 0.0015)' He < 0,001 0.02 a Except for CF,, the relative standard deviations of the least-squares-fitslopes of the Stern-Volmer plots were always less than 10%. Relative rate of quenching of NO set to 1.07for comparison with this work. Rough estimate. See text.
3
indicate that quenching of Cd(lP1) by helium, even at 300 Torr, is negligible. The main difficulty is establishing absolute kQ values for various quenchers is that h, must be estimated using a theory of radiation imprisonment. This problem is discussed below. However, relative values of may be determined with no difficulty, since there is theoretical and experimental evidence that h, 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.lOJ1Data for quenching of 2288-A fluorescence for a range of gases was shown to fit the Stern-Valmer expression, and representative plots of I o / I vs. [Q] are shown in Figure 1. Relative values of kQ determined from data such as this are showfi in Table I. Slopes of the Stern-Volmer plots were determined by a least-s uares computer routine. The fluorescent intensity a t 2288 , 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.
x
1476
The Journal of Physical Chemistry, Vol. 82, No. 13, 1978
0 00
0 50
IO0
I50
2 00
1/[q (torr-') Figure 2. Variation of CdH concentration with pressure of HP. See text. Delay time, 40 ps. [CdH], is the extrapolated value of [CdH] as [H2] approaches infinity.
It is also possible to use the formation of a long-lived product, such as CdH, as a "marker" for determining the relative rate of quenching of Cd(lP1): Cd('P,) t H,
k
2 CdH t
H
ib
-CdtHtH k
-% Cd(3PJ) t H,
--kid
Cd('S,)
+ H,
k
C d ( 3 P ~+) H, --% CdH + H
zb
@d('S,) t H,
It is known from other work that >90% of any Cd(3PJ) produced in the collisional deactivation of Cd(lP1) by Hz will be quenched via steps 2a or 2b at pressures of H2 of 0.5 Torr or higher.5 It is also demonstrated below that less than 15% of the quenching collisions between Cd(lP1) and Hz produce Cd(3PJ).6 Thus, the total yield of CdH, measured near the end of the 40-ps excitation pulse, can be used independently to obtain kQ/k,, where in this case, kQ = hi, k l b 4- hi, kid: [CdHI- k - 1 t A (11) [CdHI k,[H*I
+
+
where [CdH], is the extrapolated maximum value of [CdH] when l/[CdH] is plotted vs. l/[HZ], at [H,] L 0.5 Torr. A plot which illustrates the validity of eq I1 is shown in Figure 2. From the slope of this plot, a value of k,/kQ = 0.89 Torr was obtained, as compared to k,/kQ = 0.81 Torr from the slope of the plot in Figure 1. It can be shown that direct 3261-A excitation of Cd(3PJ) at 230 "C, amounting to only a few percent of the Cd(lP1) produced, could account even for the small difference in these two values, since the CdH produced from [H,] I 0.5 Torr would add a constant increment to the apparent [CdH] observed. It is also known that CdH production is a more important exit channel in the quenching by Hz of Cd(3PJ) than of Cd(1P1).6 As an even more independent check on the validity of the relative rates of quenching determined by this method, it is possible to compare the ratio of the quenching rates for Nzand NO determined in the current study with that measured in these laboratories in steady-state flow tube
W. H. Breckenridge and A.
M. Renlund
experiments, wherein the fluorescence of excited NO(A2Z+) produced in the quenching of Cd(lP1) by NO was used as a probe of relative Cd(lP1) concentrationsagThe ratios for Nz vs. NO quenching for the two quite different techniques, 0.60 and 0.56, are obviously in excellent agreement. Absolute Quenching Cross Sections. We believe it is important to place the relative quenching rates obtained here on an absolute scale, so that the resulting quenching cross sections can be compared with those determined by other workers for the quenching of Cd(lP1) and the other excited atomic states. The calculation of k, under our conditions has been made using the theory of Holstein,12 as modified by Walsh13 to account for simultaneous Doppler, Lorentz (pressure), and Heisenberg (natural) line broadening. The theory has been shown to predict accurately the effective fluorescence decay rate of Hg(3P1) under conditions of Doppler broadening and high opacity.14 The experiments reported here were certainly in the high-opacity region, since the opacity (kor)is 155, where ko is the maximum absorption coefficient of the 2288-A absorption line profile under Doppler-broadening conditions in the vapor pressure of cadmium at 230 "C, and r = 0.25 cm is equal to the internal radius of the 80-cm long reaction vessel. The Walsh theory should be applied to our experimental conditions with some caution, however. In the first place, as Van Volkenburgh and Carringtonlo have predicted for an infinite slab geometry, the calculated escape factor for real-time decay of imprisoned resonance radiation will show little dependence on the point of fluorescence detection, but for steady-state conditions the effective rate of escape of fluorescent photons against which the rate of quenching is measured in a Stern-Volmer experiment can vary markedly with detection geometry at high opacity because of sharply skewed distributions of excited atoms in the fluorescence vessel. However, the Van Volkenburgh-Carrington predictions are based on an idealized Doppler line shape for the lamp emission, identical with the assumed Doppler absorption line shape in the fluorescence vessel. If the lamp exciting line is severely reversed, as the 2288-A line from our flash lamp undoubtedly is, the distribution of excited states will be much more uniform because excitation will be the result of lamp photons with frequencies in the low-opacity wings of the absorption profile, particularly with the more slowly varying Lorentzian function which dominates the absorption wings under the pressure-broadened conditions of our experiments. This should be even more true for the symmetric excitation lamp geometry of our infinite-cylinder fluorescence vessel (i.e,, the flash lamp surrounds the fluorescence vessel coaxially), Boxall, Chapman, and Wayne have recently reported experimental evidence in a high-opacity system with a reversed lamp source.ll It was proved experimentally that there was no difference between the effective imprisonment escape factors in pulsed and steady-state measurements of Ar(3P,) quenching. Predictions based on the Walsh model of the effective escape factors were also shown to be correct at high opacities.ll The Walsh theory should, therefore, be generally applicable to the calculation of k, in our system. There is also little error in applying the infinite cylinder appr~ximationl~ to our 0.5 cm X 80 cm reaction vessel. The major uncertainty in the calculation, however, is that the theory has to our knowledge never been tested experimentally when Lorentzian line-broadening contributions are major due to natural and buffer-gas collisional effects. Walsh did demonstrate that the theory was able
Quenching of Cd('P1) by Several Molecules
The Journal of Physical Chemistry, Voi. 82, No. 13, 1978
TABLE 11: Absolute Cross Sectionsa for the Quenching of Cd('P,) by Various Gases quenching gas
this work (503 K ) b
i-C4Hlo n-C4H10
155
CO,
co
N2 3"
D2 HD H2
CAY"'
ref 20 (553 K)
ref 22 (368 K)
2 67
147 86 86 56 52 48 30 27 24 17 (- 0.15) < 0.03
150 138 50 58 47 11
24
1.1 0.30 a u =~kQ/U;U = [8kT/np]''Z,the mean Boltzmann speed. For this work (column l), k Q is calculated using Absolute eq 1,with k , = 9.7 x lo6 s - * (see text). uncertainty in these cross sections is estimated to be * 25%. He
to fit the experimental data for Hg(3Pl) decay up to -1 Torr of Hg vapor, where collision broadening would definitely be expected to play a role; but our conditions involve 300 Torr helium buffer gas and an additional Lorentzian contribution to the line shape due the very s,15 so that short natural lifetime of Cd(lP,), 1.66 X Lorentzian broadening plays a dominant role in the imprisonment process. Of some importance in the Walsh theory is the choice of the line-broadening parameter for the Cd(lSo lP1)transition in helium. We have chosen a value of 18 A2 for the collision broadening cross section (essentially the square of the diameter of the effective collision complex) from two separate measurements of the broadening cross section of the Hg(lSo 3P1)transition in helium.16 Using the Walsh theory, then, an effective lifetime of Cd(lP1) at 230 "C in our fluorescence vessel, with [Cd] = 1.79 X lo9 Torr and [He] = 300 Torr, is 1.03 X low7s. It is important to point out that the imprisonment lifetime calculated is not critically sensitive to the choice of collision cross section in this opacity range. A factor of 2 variation in the choice of pressure-broadening cross section would result in a change in the calculated lifetime of only -25%. It should also be noted that hyperfine and isotopic splitting of the 2288 A (lS0 'P1) line are much smaller than even the Doppler-only width of the line at these temperatures,15J7 and therefore have no effect on the calculation (in contrast to the 3261.4 (ISo 3P1)line, where the splitting is comparable to the Doppler ~ i d t h ) . ~ J ~ J ~ Using the effective lifetime of Cd(lP1) calculated, it is possible to compute absolute cross sections for the quenching of Cd(lP1). These are shown in Table 11,along with values obtained by others. To obtain further evidence for the validity of the theoretical treatment of the dependence of the effective imprisonment lifetime of Cd(lP1) on the pressure of added bath gas, and to ascertain the extent to which helium may act as a quencher of Cd(lP1) at 300 Torr, an Hz quenching experiment was also conducted a t 100 Torr of helium. The theory was used to calculate a value of k, = 5.85 X lo6 s-l, corresponding to an effective Cd(lP1) lifetime 70% longer than at 300 Torr. The quenching cross section obtained from the data at 100 Torr of helium was -20 A2 compared to 24 A2 obtained at 300 Torr of helium, which is reasonably good agreement. Total consistency a t the two helium pressures would re-
-
-+
-
-
1477
quire that the effective (imprisoned) lifetime vary less rapidly with total pressure of helium than the theory predicts. The experiment does show that quenching of Cd(lP1) by 300 Torr of helium under our conditions is essentially negligible. Experiments were also performed with argon as a buffer gas at 100 and 300 Torr, with H2 again the quenching gas. Effective radiative lifetimes at 100 and 300 Torr were and 0.93 X s, respectively. calculated to be 1.55 X Calculation of the quenching cross sections for Hz on the assumption of negligible quenching by Ar results in values of 23 and 14.0 A2 at 100 and 300 Torr of Ar, respectively. In this case, quenching by Ar may not be negligible. The best overall fit of the data with that of the helium experiments is obtained if it is assumed that argon at 300 Torr is quenching about 30% of Cd(lP1) under these conditions, altering the calculated quenching cross sections for H2 to 28 and 21 A, respectively. A better fit would again require that the imprisonment lifetime vary less rapidly with added buffer gas pressure than predicted by the theory. In any case, our data cannot be reconciled with that of Phillips and co-workers,20who report similar cross sections for the quenching of Cd(lP,) by Ar and He which are at least a factor of 5 larger than the upper limit for argon obtained in this work. E x i t Channels i n t h e Quenching of Cd('P1). Because of the relatively high energy of excited Cd(lP1), 125 kcal/mol, a variety of exit channels are available for physical or chemical dissipation of electronic energy on collision with most quenching molecules. In several cases, it has been possible with the resonance-radiation flash photolysis technique to obtain evidence for the importance of the various exit channels. A. Production of Cd(3PJ)by Collision-Induced I n tersystem Crossing. The spin-forbidden process Cd(lP1) M C d ( 3 P ~ ) M* is remarkably efficient for several quenching gases, in some cases even when quite exothermic spin-allowed chemical channels are available (Le., for the alkane hydrocarbons). We have determined the relative yields of CdCSPJ)in the quenching of Cd('P1) by several gases. Relative concentrations of Cd(3PJ) were estimated by measuring the emission intensity at 3261 A due to Cd(3Pl)fluorescence, in the same experiments in which the quenching of 2288 A fluorescence was measured. The experimental observations were as follows. With 300 Torr of He and no quenching gas, the intensity of 3261-A fluorescence from the excitation of Cd(3Pl) was small but measurable. Addition of -0.2-0.7 Torr of N or the alkanes resulted in up to 30-fold increases in 3261-1 fluorescence intensity. Slight increases in 3261-A fluorescence intensity were also observed with the addition of similar pressures of CO, CF4, and NH,. Addition of all other gases caused a decrease in 3261-A fluorescence. As little as 0.02 Torr of Hz,for example, caused a measurable decrease in the 3261-A fluorescence intensity. Again, scattered light from the flash lamp at 3261 A was shown to be negligible since the fluorescence could be completely eliminated by sufficient pressures of quenching gas. Measurements of Cd(3PJ) in absorption gave results consistent with the 3261-w emission data. By correcting for incomplete quenching of Cd(lP1) under these conditions, using the data from which the quenching rates in Table I were computed, it is possible to calculate the relative yield of Cd(3PJ) for those quenchers for which quenching of Cd(3PJ) was negligibly slow5at the pressures of quencher used, i.e., N2, alkanes, CF4, and NH3. The measured fluorescence at 3261 A was then normalized to
+
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+
1478
W. H. Breckenridge and A. M. Renlund
The Journal of Physical Chemistry, Vol. 82, No. 13, 1978
TABLE 111: Relative Yields of "PJ States in the Quenching of the 'P, States of Cd and Hg re1 yield of re1 yield of quenching gas Cd( 3pJ H ~ ( ~ P ~ 100 100 NZ i-C4H,,, 110 * 40 25 CHa 7 3 i- 25 90 i 33 30 Ck,$ 15r 3 25 co CF, 10 * 5 (- 20) 3 % 1.5 6The most likely primary sensitization step is Cd('P,)
+ RH
-+
Cd
+RtH
with further reactions of R and H producing alkenes and/or H2. Direct identification of this exit channel requires application of other techniques not available for this study. No evidence for a decrease in 3261-A fluorescence intensity with the number of excitation flashes was observed for any quenchers except the alkanes. D. Production of CdF. Production of CdF was observed in absorption in the quenching of Cd(lP1)by SF6. Because the measurements above have shown that production of Cd(3PJ) by collision-induced intersystem crossing is very inefficient for SF6, the CdF results from a primary exit channel Cd('P,) t SF, SF, + CdF -f
Quenching of CdPPJ) by SF6 is also known to produce CdF.6 Comparison of experiment8 with SF6 in pure helium and in pure N2 (where all Cd('P1) should be deactivated) show that the production of CdF is at least twice as efficient in the quenching of Cd(3PJ) than of Cd(lP1), possibly indicating the existence of a direct SF6-F bond cleavage exit channel for the quenching of the higherenergy Cd(lP1) state. Note that formation of CdF by reaction of ground-state Cd('So) with SF6 is -20 kcal/mol endothermic, and SF6 appears to be quite stable in the presence of hot cadmium vapor. The production of CdF has also been detected in qualitative experiments with PF3 and C4F8 (perfluorocyclobutane),but there is insufficient information in these cases to rule out Cd(3PJ) formation and subsequent reaction to form CdF. It is known that Cd(3PJ)is quenched very efficiently by PF3, and that CdF is formed in the quenching process.21 There was no CdF observed in the quenching of Cd(lP,) by CF4.
Discussion As seen in Table 11, our cross sections are in fair agreement with those obtained by other workers using different experimental methods.8~20~22 It is obvious that except for He and Ar all the gases studied are relatively effective quenchers, with rates corresponding to quenching of Cd(lP1) on the order of every hard-sphere collision. Similarly indiscriminate and rapid quenching by a variety of molecules has also been observed for several other atomic excited ~ t a t e s , l . ~i.e., ~ -Ad3P2), ~~ Kr(3P2),Xe(3P2), Hg(lP1), K(5p 2P), and Rb(6p 2P). In contrast, excited states such as Hg(3PJ,27Hg(3P1),3Cd(3PJ),6Mg(3P~),2s and the first 2Presonance states of all the alkali metals26~29~30 are quenched by polyatomic molecules with cross sections which can vary by several orders of magnitude. It is interesting to note that for these metal and inert-gas cases, the states for which there is indiscriminate quenching are either high in energy or have atomic excited states lower in energy. Selective quenching thus seems for these atoms to be associated with the lowest atoniic excited state of the atom, so long as the electronic energy is sufficiently small
Quenching of Cd(’P1) by Several Molecules
The Journal of Physical Chemistry, Vol. 82, No. 13, 1978 1479
TABLE IV: Comparisons of Cross Sectionsa of Quenching of Cd(lP,) and C d ( 3 P ~by ) Several Gases
2.200
“QI
i-C4Hlo
SF, NO
co, co NZ
“3
DZ
HD HZ CF.4 Ar He a UQ
speed.
Cd(’P1) (503 K)
Cd(3 P ~ ) b (553 K )
155 147 86 86 56 52 48 30 27 24 17 -0.15 < 0.03
(< 0.01) 7.2 57 38 5.3 0.024 0.14 6.8 12.3 12.3 ( 198 kcal. Finally, note that the cross section for quenching of Li(2p *P)by CzH6 is only -2 A2, even though the A parameter is bound to be nearly the same as for Cd(lP1) since the two excited atomic states have identical ionization potentials. The hard-sphere atomic diameter of Li is -3.0 A,51952also virtually identical with that of Cd and Hg, so that the effective radii (and thus the onset of M*-RH repulsion) should be roughly the same for Li(2p 2P)and Cd(lP1). The low cross section for Li(2p 2P)quenching by C2H6 is therefore good evidence that charge-transfer coupling is not operative in the collisional interaction of any of these excited states with simple saturated hydrocarbons. Some other coupling scheme, perhaps of the "golden-rule'' type mentioned above,1141*44 must then be invoked to explain the very large cross sections for the quenching of Cd(lP1) by alkanes. In this regard, the only exit channel available for quenching of Li(2P)by C2H6 is the formation of vibrationally excited ground-state alkane molecules, since even the formation of LiH + C2H, is endothermic. Note, however, that the cross sections for CO, COz, and SF6 parallel quenching of Li(2p 2P)by Nz, those for the quenching of Cd(lP1) by the same species and thus provide added evidence for the charge-transfer mechanism for molecules with positive or only moderately negative electron affinities. Collision-Induced Production of Cd(3P~).Measurements of the absolute branching ratios for Cd(3PJ) formation in the quenching of Cd(lP1) by N2 and the alkanes were not possible with the present system, but it seems rather unlikely that maximum similar relative branching ratios determined for so many gases (N2,CH4, C3H8,and i-C4H10)could result from fortuitously similar, but small, branching ratios. It is far more likely that the branching ratios for Cd(3PJ) formation for these gases approach unity, or are a t least greater than 0.5. There appears to be no obvious generalized explanation of the efficiency by which such disparate quenchers as the alkanes and N2 deactivate Cd(lP1) to Cd(3PJ). For N2 and CO, the charge-transfer mechanism at least provides a coupling mechanism by which Cd(3PJ) and vibrationally excited diatomic could be produced in competition with the Cd(lSo) exit channel (ha vs. kh above). Some kind of analogous coupling scheme for the alkanes is more difficult to conceptualize, particularly in view of the previous arguments that charge-transfer interactions are likely to be of minor importance in the quenching of Cd(lP1) by alkanes. It is possible, of course, that an unknown mechanism which allows "golden-rule'' coupling of Cd(lP1) R H to the continuum of Cd('So) + R + H states could also allow coupling to Cd(3PJ) RH*, where RH* represents an accessible distribution of rovibronic states of ground-electronic-state RH. One could also postulate a long-lived excited R-Cd-H intermediate (an alkyl metal hydride) which lives sufficiently long to facilitate decomposition as well as curve crossing, analogous to the complexes through which it is thought that O(lD) is deactivated by several molecular q ~ e n c h e r s . ~ ~ Whatever the mechanism(s), similar behavior has been observed for Hg(lP1)-efficient production of Hg(3Pl) from quenching of Hg('P1) by alkanes, N2,and C0.23 (See Table 111.) Although relative to Nz, alkanes seem to produce the 3 Pstate ~ less efficiently for Hg than for Cd, it should be pointed out that there is an uncertainty in the Hg(3P1)data which is not present in the Cd(3PJ) data. For Cd(3PJ), the measured distribution of Cd(3Po,1,2)is that of an equili-
+
+
1483
brated set of J states which decay by Cd(3P1)fluorescence under the conditions of 300 Torr of helium and few Torr of R H or N2 used for these measurements. Absorption experiments proved this for N2, the alkanes, and CO. For the Hg(lP1) case, however, large amounts of Hg(3Po! or Hg(3P2)could be produced which would never result in a fluorescent photon from Hg(3P1).65 The values listed in Table I11 for Hg(3P1)are therefore only valid if quenching by all the species produces the same distribution of Hg(3Po,1,z)states as does Nz-but the general trends seem to parallel the Cd results. Accurate absolute branching ratios for 3P0,3P1,and 3P2formation for both Cd and Hg would be extremely valuable in attempting to understand the mechanism of these processes. It would also seem important to determine whether the spin-forbidden lP1 3PJexit channel remains so remarkably efficient for lighter atoms with the same excited state manifold, such as Zn, Mg, or Ca, and we are currently conducting experiments which should provide more accurate branching-ratio data for a variety of metal atom excited states. The failure to observe Cd(3P~) as a major product in the quenching of Cd(lP1) by NO is somewhat surprising, since the exit channel in this case is not even spin forbidden8 It is possible that analogous to CO quenching, the production of ground-state products is also the dominant exit channel in the deactivation of Cd(lP1) by NO, although a spin-forbidden channel, Cd(lSo) + NO(a 4rIJ,cannot definitely be ruled ouLs Electronic energy transfer to form NO(A22?) has previously been shown to occur8 even though the process is endothermic, but the branching ratio is 50.04.
-
Acknowledgment. Acknowledgment is made to the donors of the Petroleum Research Fund, administered by the American Chemical Society, and to the National Science Foundation, for support of this research. We are also grateful to the University of Utah Research Committee for a graduate fellowship awarded to A.M.R. References and Notes (1) D. L. King and D. W. Setser, Annu. Rev. Fhys. Chem., 27, 407 (1976). (2) J. G. Calvert and J. N. Pitts, "Photochemistry", Wiley, New York, N.Y., 1966. (3) R. J. Cvetanovic, frog. React. Kinet., 39 (1964). (4) E. W. R. Steacie, "Atomic and Free Radical Reactions", 2nd ed, Voi. 1 and 2, Reinhoid, New York, N.Y., 1954. (5) W. H. Breckenridge, T. W. Broadbent, and D. S. Moore, J . Phys. Chem., 79, 1233 (1975). (6) W. H. Breckenridge and A. M. Reniund, J . fhys. Chem., following paper in this issue. (7) W. H. Breckenridge and A. M. Renlund, to be submmed for publication. (8) W. H. Breckenridge and J. FitzPatrick, J . fhys. Chem., 80, 1955 (1976). (9) W.H.'Breckenridge, R. P. Blickensderfer, and J. FitzPatrick, J. fhys. Chem., 80, 1963 (1976). 1 G. V. Van Volkenburgh and T. Carrington, J. Quant. Spectrosc. Radiat. Transfer, 11, 1181 (1971). M. J. Boxall, C. J. Chapman, and R. P. Wayne, J . fhotochem., 4, 281, 435 (1975). T. Holstein, Phys. Rev., 72, 1212 (1947); 83, 1159 (1951). P. J. Walsh, fhys. Rev., 116, 511 (1959). A. V. Phelps and A. 0. McCoubrey, fhys. Rev., 118, 1561 (1960). A. Lurio and R. Novick, fhys. Rev. A , 134, 608 (1964). A. C. G. Mitchell and M. W. Zemansky, "Resonance Radiation and Excited Atoms", Cambrldge University Press, Cambridge, 1971, p 171. N. Heydenburg, Phys. Rev., 43, 640 (1933). P. Brix and A. Steudei, Z. fhys., 128, 260 (1950); M. N. McDermott and R. Novlck, fhys. Rev., 131, 707 (1963). I t should be noted that In an interesting numerical study of radiation trapping by Phillips (L. F. Phillips, J . Photochem.,5, 277 (1976)), the absorption coefficient for the Cd 2288-A line was divided by 3 in the same manner as was the 3261-A line, apparently in order to make a rough correction for hyperfine splitting. While this may give approximately correct trapping tlmes for the 3261-A line, such a procedure is incorrect for the 2288-A line. However, the true trapplng times for the Phillips study can be obtained by dividing the cadmium vapor concentration by a factor of three for each trapping time calculated.
1404
The Journal of Physical Chemistry, Vol. 82, No. 13, 1978
W. H. Breckenridge and A. M. Renlund (42) A. C. Vikis and H. C. Moser, J . Chem. fhys., 53, 2333 (1970). (43) H. E. Gunning, J. M. Campbell, H. S. Sandhu, and 0. P. Strausz, J . Am. Chem. Soc., 95, 746 (1973). (44) G. C. Marconi, G. Orlandi, and G. Poggi, Chem. fhys. Left., 40, 88 (1976). (45) R. A. Holroyd and T. E. Pierce, J . fhys. Chem., 68, 1392 (1964). (46) A. B. Callear and J. C. McGurk, J . Chem. SOC., Faraday Trans. 2, 68, 289 (1972). (47) H. Hunziker, private communication. (48) A. C. Vikis and D. J. LeRoy, Can. J . Chem., 51, 1207 (1973). (49) B. L. Earl and R. Herm, Chem. fhys. Lett., 22, 95 (1973). (50) J. T. Waber and D. T. Comer, J. Chem. fhys., 42, 4116 (1965). (51) J. C. Slater, J. Chem. fhys., 41, 3199 (1964). (52) J. 0. Hirschfelder, C. F. Curtiss, and R. B. Blrd, "Molecular Theory of Gases and Liquids", Wlley, New York, N.Y., 1964. (53) SF,: R. N. Compton and C. D. Cooper, J . Chem. Phys., 59, 4140 (1973); M. M. Hubers and J. Los, Chem. fhys., 10, 235 (1975). COP: R. N. Compton, P. W. Reinhardt, and C. D. Cooper, J . Chem. Phys., 63, 3821 (1975). NO: P. D. Burrow, Chem. fhys. Lett., 26, 265 (1974). N:, D. Mathur and J. B. Hasted, J. fhys. 6,10, L265 (1977). CO: G. J. Schulz, Rev. Mod. fhys., 45, 378 (1973). (54) E. R. Fisher and G. K. Smith. ADD/. Oot.. 10. 1803 (1971). (55) J. Costeilo, M. A. D. Fluendy,' a i d K. P. Lawley, Faraday Discuss. Chem. Soc., 62, 291 (1977). (56) K. A. Kohler, R. FeNgen, and H. Pauly, fhys. Rev. A , , 15, 1407 (1977). (57) S. J. Riley and D. R. Herschbach, J. Chem. fhvs.. 58. 27 (1973). (58) S. M. Freund, G. A. Fisk, D. R. Hershbach, and W. Klemperer, j . Chem. fhys., 54, 2510 (1971). (59) D. S. Y. Hsu and M. C. Lin, Chem. Phys. Lett., 42, 78 (1978). (60) F. A. Cotton and G. Wilkinson, "Advanced Inorganic Chemistry", Interscience, New York, N.Y., 1972, p 711. (61) G. Karl, P. Kruus, and J. C. Polanyi, J. Chem. fhys., 46, 224 (1967). (62) A. C. Vikis and D. J. LeRoy, Chem. fhys. Lett., 21, 103 (1973). (63) J. Berkowitz, W. A. Chupka, and T. A. Walter, J . Chem. Phys., 50, 1497 (1969). (64) J. Tuiiy, J . Chem. fhys., 62, 1893 (1975); R. G. Shortridge and M. C. Lin, ibid., 64, 4076 (1976). (65) R. Burnham and N. Djeu, J . Chem. fhys., 61, 5158 (1974).
(20) P. D. Morten, C. G. Freeman, R. F. Claridge, and L. F. Phillips, J . fhotochem., 3, 285 (1974). (21) A. M. Renlund, unpublished results. (22) R. Pepped, Z. Naturforsch. A , 25, 927 (1970). (23) V. Mahaven, N. N. Lichtin, and M. 2. Hoffman, J. fhys. Chem., 77, 875 (1973). (24) A. Granzow, M. Z. Hoffman, and N. N. Lichtin, J . fhys. Chem., 73, 4289 (1969). (25) B. L. Earl and R. R. Herm, J. Chem. Phys., 60, 4568 (1974). (26) I. N. Siara and L. Krause, Can. J . fhys., 51, 257 (1973); M. Czajkowski, L. Krause, and G. M. Skandis, ibid., 51, 1582 (1973). (27) A. Callear and J. McGurk, J . Chem. Soc., Faraday Trans. 2, 69, 97 (1973). (28) R. P. Blickensderfer, W. H. Breckenridge, and D. S. Moore, J. Chem. fhys., 63, 3681 (1975). (29) S. Lln and R. E. Weston, Jr., J . Chem. Phys., 65, 1443 (1976). (30) P. L. Lijnse, "Review of Literature on Quenching, Excitation, and Mixing Collision Cross-sections for the first Resonance Doublets of the Alkalis", Report 398, Fysisch Laboratorium, Rijksuniversiteit Utrecht, The Netherlands. (31) J. C. Slater and J. G. Kirkwood, fhys. Rev., 37, 882 (1931). (32) Landolt-Bornstein, "Zahlenwerte und Funktionen", Vol. 6, Aufrage I, 1 and 3; M. J. Bridge and A. P. Buckingham, Roc. R. SOC. London, Ser. A , 295, 334 (1966); A. B. Tipton, A. P. Dean, and J. E. Boggs, J . Chem. fhys., 40, 1144 (1984). (33) L. N. Shabouna, Opt. Spectrosc., 27, 205 (1969). (34) H. A. Hyman, Chem. fhys. Lett., 31, 593 (1975). (35) B. L. Earl, R. R. Herm, S. M. Lin, and C. A. Mims, J . Chem. Phys., 56, 867 (1972). (36) J. R. Barker and R. E. Weston, Jr., J. Chem. fhys., 65, 1427 (1976). (37) L. G. Piper, J. E. Velazco, and D. W. Setser, J. Chem. fhys., 59, 3323 (1973). (38) E. A. Gisiason and J. G. Sachs, J . Chem. fhys., 62, 2878 (1975). (39) E. Bauer, E. R. Fisher, and F. R. Gilmore, J. Chem. fhys., 51, 4173 (1969). (40) P. L. Lijnse, J. Quant. Spectrosc., Radiat. Transfer, 14, 1143 (1974). (41) J. E. Velazco, J. H. Kolts, and D. W. Setser, J . Chem. Phys., 65, 3468 (1976).
Reaction of Excited Cadmi~rn(~P,)and Cadmium('P,) Atoms with Hp, HD, and DP. Quenching Cross Sections and CdH(CdD) Yields W.
H. Breckenridge*+ and Anita M. Renlundt:
Department of Chemistry, University of Utah, Salt Lake City, Utah 84 I12 (Received February IO, 1978) Publication costs assisted by the Petroleum Research Fund
Cross sections and primary CdH(CdD) yields have been measured for the quenching of Cd(3PJ) and Cd(lP1) by Hz, HD, and Dz. Cross sections for Cd(lPJ are larger and show less isotopic selectivity than those for the quenching of Cd(3P~).The cross sections for the quenching of Cd(3P~) are not markedly temperature dependent, with Arrhenius activation energies 52.0 kcal/mol. The total yield of CdH + CdD in the quenching of Cd(lP1) by the isotopic hydrogens is 50.50 of the CdH + CdD yield in the quenching of Cd(3PJ),which in turn is near unity. There is no isotope effect on the total CdH + CdD yield in Cd?P& or Cd(lP1) quenching. The product ratio of CdD to CdH in the quenching of Cd(3PJ) by HD is 1.9 k 0.2, but in the quenching of Cd('P1) by HD is 1.2 f 0.1. The isotope effects on cross sections and relative CdD/CdH yields for Cd(3P~) quenching are discussed in terms of an adiabatic chemical reaction in which Cd(3Pl) attacks Hz, HD, or Dz side-on, and where the production of CdH or CdD is either thermoneutral or slightly endothermic. It is postulated that Cd(lP1) quenching by Hz involves an efficient nonadiabatic surface crossing, and that a charge-transfer potential surface is likely involved due to the lower ionization potential of Cd(lP,). It is also suggested that with the higher-energy Cd('P1) state there may be no constraint to side-on attack of hydrogen, perhaps accounting for the lack of isotopic specificity as compared to the reactions with Hz of Cd(3P1)or the 3P states of Hg. Introduction In previous papers,112 we have reported the determination of absolute cross sections and major exit channels
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Camille and Henry Dreyfus Foundation Teacher-Scholar, 1973-1978. Department of Physical Chemistry, Cambridge University, Cambridge, England. 0022-3654/78/2082-1484$01.00/0
for the quenching of the two lowest-lying excited states of the Cd atom, C d ( 3 P ~and ) Cd(lPl), by several gases. Here we describe a more detailed comparative study of the quenchinn of these states by the simplest of molecular quenchers Hz, HD, and Dz. -Using the technique of resonance-radiation flash photolysis, it has been possible to measure the temperature dependence of the cross Sections for Cd(3PJ quenching, to measure the different relative 0 1978 American
Chemical Society
