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Collisional metastability of high rotational states of CN(X2Σ+, = 0)
Volume 118, number 1
CHEMICAL
PHYSICS
12 July 198.5
LETTERS
COLLISIONAL METASTABILITY OF HIGH ROTATIONAL STATES OF CN(X ‘2 +, d’ = 0) * S. HAY, F. SHOKOOHI,
S. CALLISTER
Chemrsrry
of Southern
Depcutment.
Uniwrsr@
and C. WI-lTIG
Cahfotnicq
LDs AngeL,
CA 90089-0484.
USA
Rcceivcd 14 February 1985; in final rorrn 10 May 1985
CN(X ‘z +. u”= 0) high rotational stales relax slowly via 300 K collisrons with Ar and Kr_ Relaxation decrease with increasing relation. and the partially relaxed dlstnbutions are bimodal. with low N” thcrrnalkd (300 K), and N”= 80 unrelaxed afier loo0 collisions. Relaxation by N,, CO, and Xe is similar to Ar and Kr, but more efficient. He and NO remove many quanta in a smgle co&ion
1_ Intro4hction It is often assumed that the rotatronal degrees of freedom of small gaseous molecules are efticrently thermalized by collisions with inert species. The spacings between adjacent levels are usually small compared to kT, and it is not uncommon for rotational energy transfer cross sections to exceed those for hard-sphere collisions [ 11. However, this rule of thumb is not universal_ For small moments of inertia and/or large rotational quantum numbers, the spacings between adjacent levels can be larger than kT, and rapidly rotating molecules may appear more isotropic durmg a collision than their slowly rotating counterparts With hydrogen halides, for example, high rotational states can survive many collisions with Ar [2-71, and this propensity can help explain the operating characteristics of hydrogen halide rotational lasers 151. With heavrer nuclei, the same propensity persists, and Pntchard et al [8-101 and Bergmann et al. [ll] have combined careful deactivation measurements with theoretical models, thereby providing quantitative bases for predicting rotational exchange for dlatomlcs. By using photodissociation to produce rotationally excited diatomics, ground electronic-state species can be prepared in rather high rotational states. For * Research
6
supponed by the Nahonal Scicna Foundation
example, the 193 run photodissociation of BrCN produces CN with little electronic or vibrational excitation (~1% in the A 211 state [12], 66% wrth U” > 1) [12], and withN” up to 86, corresponding to as much as 14000 cm-l of rotational energy. With room-temperature samples, the spacings between adjacent rotalional states withN” z 80 are comparable to the average translational energy, and based on previous research, one expects these h@ZV” to be metastable with respect to collisions with small spherically symmetric species [13-G] _ In this cornnunication, we report the direct observation of this metastabrlity, and note that with certain partners a large number of collisions are required in order to thermalize the CN rotational degrees of freedom_ For example, the average rotational energy, UZ,,,), of the nascent CN produced by the 193 nm photodissociation of BrCN is 6100 cm-l, and thousands of collisrons are required in order to relax (Er,t) down to -00 cm-I= using Ar or Kr buffers. In addition, the partially relaxed distributions are very bimodal, with the low N” therm&red at 300 K, and the lughest N” retaining their nascent distribution.
2. Experimental The experimental arrangement has been described in detail previously [ 131. Briefly, 193 run excimer la0 009-2614/85/S (North-Holland
03.30 0 Elsevier Science Publishers B.V. Physics Publishing Division)
Volume 128, number 1
CHEMICAL
PHYSICS LETIJZRS
ser photolysis of BrCN is followed by laser induced fluorescence (LIF) detection of CN (O-14 cm-l resolution), and signals are digitized and averaged until suitabIe Sfnr is achieved_ A narrow bandpass interference filter centered at 388.5 nm (20 nm fwhm) is-used to lsolate the B 2Z+-X 2Z+ emission. Relative populations are obtained from the spectra following careful procedures which have been described in detail previously [ 161. For example, when the spin-rotation components are not fully resolved, the line intensities are taken from the areas of the absorption lines rather than the peak heights. Ln cases of overlapped absorption lines (e-g , the b~dhead region) or perturbed rotational levels, the relative populations are obtained by fitting the overlapped spectra and/or extrapoIation. BrCN (Aldrich, 97%) is purified by vacuum distillation prior to experiments, NO (Matheson, >99%) is subJetted to repeated trap-to-trap distillations, He, AI, P-branch
12 July 1985
and Kr [Spectra Gases, ultrahigh purity), Xe (Spectra Gases, >!39.99%), N2 (M-G. Industries, research grade, 99.9995%) and CO (Matheson, x!? 99%) are used without further purificatron.
3. Results The spectra shown . I fig 1 reflect the metastability of the high N” with respect to colhsional deactivation by Ar The upper trace 1s for nascent CN, and nnmediateiy below it II; the spectrum of a 300 K sarnpie_ Notice the prom~ent Doppler broadening in the spectrum of nascent CN, and the unequal populations III the spur-rotatron doublets Trace (C) shows the spectrum obtained with 100 Torr of Ar and a 300 ns delay between the photolysis and probe lasers, the high N” are split by spin-rotation interactron and per-
W-
Fig. 1. LIFVspectra of CN(X2Z+, u” = 0). following the 193 nm photolyds of BrCN. In {A), the sample COC~GIIS20 mTon BICN and the delay between the photolysis and analysis lasers in 300 ns The w-m displays nascent CN rotational excitation; the mme spectrum is obtained.with lower BICN pressures and skirter delays Zn (B), 10 aTon BICN, 100 Ton He, and a 500 ns delay provide a completely th ‘- d .mmple (300 x;) for aunpmn purposea Trace (c) (10 mT’o= BrCN, 100 TOH Ax, 300 ns delay) showS pa&al relaxatio& with low IV” thermaked at 300 K Trace (33 isobtained from (C) by removing the low N” and expanding the vertical axis, to aid identification of the highs T-kuuzh transitions (+ denotes the perhubedN” = 60 level).
7
Volume i 18.
number
1
4
ElrCN-!==--+
12 July 1985
PHYSICS LE-ITERS
CHEMICAL
Br + CN(X*E+. vu. N’)
d
t
amA
A” A A 0
IO Torr
Ar
*
100 Torr
Ar
‘3
“934
0 1
0
‘3
bdra
noscenl
1
10
,
I 20
5
I
I
30 CN tX=E+, v-=0,
I
40 N’)
I
I
50
ROTATIONAL
I
I
60
I
I
70
1
I
80
LEVELS
2. Rotational populatinn distributionsfor nascent conditions, as well as 10 and 100 Torr 01A.r (300 ns delay). The b&nodal nature of the pattially relaxed distrz%utionsis evident; notice that the extent of relaxationdecreasesmonotonically with mcrea+ Fjg.
ingN”_
-
sist even though many (=300) collisions have occurred (13]_ Since the high N” are split and spread over more levels than the lower N”, they contain a larger fraction of the population than one might infer casually from the rclatlve peak heights. For example, 60% of the population have N” 2 40 with 100 Torr Ar and 300 ns delay, corresponding to Er,,~ = 4300 cm-l_ Trace (D) is obtained from the one above it by removmg the 300 K component and expanding the scale, in order to make identifications easier. The small peaks on the short-waveleng~ side of N“ = 62 arc due to u“ = 0 R-branch and u” = 1 Pbranch transitions_ We estimate that nascent CN vibrational excitation LSweI1 withm the limits reparted previously [ 12). As in the work of Jackson et al. [ 121, we also observe that U” = 1 populations increase dunng the mitial transient phase, in which nascent translational excitation is degraded_ The extent of increase depends on the collision partner, with the heavy rare gases (& and Kr) leading to increases of the u” = 1 population of as much as a factor of 3. The rotational d~trlbutions associated with spectra such as the one shown in fig. lc are very b&nodal, and exampies are shown in fig. 2. The lower states are 8
well characterized by a 300 K B~ltv~la~ dlstnbution, and the high states show the effect of the collisional transfer rates decreasing with increasmgN”_ The distributions in fig. 2 show the collislonal metastability of the highest rotational states detected ior the present experiments. Although we presently monitor levels withN” 4 80, the VW photolysis of ICN is known to producefV”’ up to 107 [17f, having -21100 cm-l of rotatlonal energy and spacings between adjacent levels of WOO cm- 1 _The physics of such species has never been studied experimentally, and we note that by optically pumping to the B 2Z* state, isolated highN can be studied as per the methods pioneered by Steinfeld et al. [18], Pritchard et al. [S-10], and Bergmann et al. [ 1 l] in cases where rotational relaxation is more efficient in the B 2E? state than the X 2Z+ state. The distributions shown in fig. 2 indicate the main features of the relaxation of rotationally excited CN via cohzons with Ax. Orice below N” EYU 40, relaxation 1s sufficiently rapid that species are therrnaiized at 300 K on the time &ale of the measurements, Above N” = 40, however, relaxation is slow, with the highest N” showing the feast amount of relaxation,IL? 2 67
Volume 11.S, number 1
CHEMICAL
is essentially unrelaxed. Considerable relaxation of N” = 57-67 occurs during the first few collisions as demonstrated by pronounced differences between the nascent distribution and that obtamed with 1 Torr Ar [ 19]_ The high nascent velocities from BrCN photo. dissociation result in relatively efficient rotational relaxation dunng the initial transient phase [20,21], in which the hyperthermal velocities are degraded to 300 K In the present experiments, the maximum pressure was limited to 100 Torr by the flow arrangement, as well as the need to insure that CN(B 2Z*, u’ = 0) rotational levels are not appreciably quenched with N” dependent cross sections. Under these conditions, relaxation 0fN” = 80 is quite small (i.e. there is no measurable relaxation with -lo3 colbsions), and 2104 collisions may be requked in order to relax these levels efficiently. In the case of k, relaxation is slightly less efficient than AI, as can be seen in table 1. However, the qualitative behavior 1s almost identical to that shown in fig. 2, and spectra taken with 100 TOIT of Kr and several s delay show prominent highIV” populations. N2 is more efficient than Ar, and as with Kr, the qualitative behavror is almost identical to that shown in fig. 2. CO and Xe are more efficient than N2, and again the bimodal nature of the partially relaxed distribution is
BrcN 0
.
12 July 1985
PHYSICS LEmERS
---==G-
Table 1 Relaxation of the averageCN rotational energy, arot>, by
different co&sion partners The number of collisions,R=, from data tabulated in reE [22]
Collision partner
narcent 3 IG Ar
Pressure
Delay
(T-1
W
0.02 100 1.0 10
300 600 300 300
30 100
300
35 11
N2
Xe He
1.0
10 30 100 NO
0.5 1.0
300
300 300 300 300 500 500 300 300
(Elot) (cm-l)
nC
6100 4200 5300 4650 4400 4300
700 b) 37
2000
150
2850
110
370
b)
42
4900 1900 210 200
60 300 1000
3400 2400
b, e) b, c)
a) The spectrumdoes not changenotmzably at lower pxe? sums and/or shorter delays. b) At these low pressures,degradationof the na.5centtransliltional energymakes it impotibk to estimate the number of colkions. a The reactivenature of the CM/NO system makes hardsphere collision moss sections inappropriate.
Br + CN(X’E+:+.
0
100 Torr
He
0
!O Ton-
me
*
f Torr
He
CN(X'LC+. v"=O, N-1
is
caldated
ROTATIONAL
v”. N")
LEVELS
Fig 3.Rotationalpopukition distributionsfor 1, 10, and 100 Ton of He (300 ns deWI.
9
Volume 118. number 1
CHIZIICAL
PHYSICS
12 July 1985
LETTERS
conserved. In the case of He (fig. 3) there is much less bimodal character in the distributions than wrth Ar, Kr, N,, CO or Xe It appears that the high velocity and small size of He allow rt to penetrate the potenteal of rotating CN and provide enough torque to remove a number of ro tational quanta in a single en-
tional states, removing multiple quantain a single collision [9,26], and this can account for the partially relaxed distributrons having httle bimodal character_ Smce He is small and light, it can get close to rotating CN, thus providing the torque which is required for rotational exchange_ Similarly, the large nascent CN
counter_ The high quenching efficiency of Xe relative to Ar and Kr can be attributed to the chemistry of this specres. The ground potential surfaces of XeF and XeCl
translational excitahon can facilitate energy transfer with heavy species such as Ar, durmg the initial transient phase in which translations are thermahzed. This can be seen by noting that rotations are relaxed more efficiently prior to translational thermalization than
are bound by 1065 cm-l [23] and 255 cm-l [24] respectively, and based on CN’s electronegativity, we expect the XeCN bond strength to lie between these values. Thus, the mteraction energy is much stronger than one would infer from the dipole-induced-dipole
afterwards. The relaxation of rotationally excited CN by collisions with NO is very efficient, as shown in fig. 4. Many quanta .are removed in just a single collision,
contribution, and this can enhance the energy transfer cross sections. With ArF and KrF, the ground potential surfaces are essentially repulsive [25] and we anticipate no sign&ant attraction in either the Ar/CN or
and the spur-rotation more rapidly than the smce NCNO IS bound CN + NO [27], and a
Kr/CN systems. Quahtatively, collisional relaxation by the rare gases He, Ar, and Kr follows the trends reported by Brunner et al. [9] in theu study of rotational relaxation of Na,. It is known that He can efficiently relax high rota-
may form an NCNO* intermediate. In dissociating, NCNO* wdl apportion the energy m excess of reaction threshold statistically, thereby causing efficient relaxation of the initial CN R,T energy and equalization of the spin-rotation states [19]. It IS known that
BrCN v
.
*“A
_
193nm
Br + CN (XCEf,
0.5 Tom 1
Torr
states are equilibrated even rotations. This is not surptising, by 49 kcal mol-1 relative to large fraction of the collisions
Y”,
N=)
NO 43
NO
P
P
P
* n
0
20 30 CN CX’E+, v-=0,
40 N-1
ROTATIONAL
50
60
70
El0
LEVELS
Fig. 4. Rotational population distilbutions for nascent conditions, as well as 0.5 and 1.0 Torr of NO (300 us delay). Even w&h 0.5 Tom NO, many rotational quanta have been relaxed Notice the similarity between the cases of 1 Torr NO and 10 Torr He (t-&G3). 10
Volume 118. number 1
CHEMICAL
PHYSICS
CN reacts with NO at 300 K forming N2 + CO (1.2 X lo-l3 cm3 molecule-1 s-l) [28], and it is possible that some species are removed by reaction rather than melastic scattering. However, in comparing the signal intensities of thermallzed CN samples for the cases of He and NO, we note that the hyperthermal nascent CN is not consumed efficiently by NO. Perhaps the bimolecular reaction: CN + NO + N, + CO transpires via the NCNO* complex U-Icompetition with dissociation back to CN + NO reagents. In most of the experiments listed above, the BrCN pressure was lo-20 ml‘orr With added gas pressures that significantly exceed that of BrCN, the translational degrees of freedom are thermaltied rapidly, so that collisions occur at a 300 K rate. Thus, with the BrCN pressure constant, we observe changes due to the added gas, under conditions in wluch the CNBrCN collisions occur at a 300 K rate This insures that effects are due to the additive, and not an alteration of collisions between translatronally hot CN and the BrCN substrate There are several reports in the literature of 121% rotational states being relaxed inefficiently via collisions with rare gas atoms. The present results confYm
the established qualitative trends [6,14,15,27] and mdicate that such metastability might be quite pronounced under certain circumstances. For example, with 100 Torr of Kr buffer and delays of 2 ps, there is little relaxatron of levels withN” = 80. These CN molecules have experienced >1 O3 collisions with little loss of energy, and in order for them to become “therrnalized” at 300 K. it will require passage from N” 2; 80 to N” < 20, and this wrll probably require 21 O4 collisions. With even higher N” (e-g_, N” 5 107 are prepared by the VUV photodissociation of ICN [ 171).and/or lower ambient temperatures, an even greater degree of metastability is anticipated_ Thus, in interstellar media, where collisions are infrequent and VUV photolysis IS common, such metastability can play a significant role m chemicaI as welI as radiative transport processes. References J-T_ Yardley, Introducbon to molecular energy transfer (Academic Press, New York, 1980). [ 21 H.K. Haugen, W-H. Pence and S.R. Leone. J. Chem Phys 80 (1984) 1839. [ 31 J.C. Polanyi and K.B. Woodali. J. Chem. Phya 56 (1972) 1563.
[l]
LETTERS
12 July 1985
[41 J.A. Barnes M. Keil, RE. Kutina and J.C PolanyI, J_ Chem_ Phys 72 (1980) 6306. 13 0-D. Krogh and G.C. Punentel, J_ Chem Phyr 67 (1977) 2993. 161 D.J. Bogan, D.W. Sctrer and J.P. Sung, J. Phys Chem. 81(197T) 888. J. Chem. Phys 69 (1978) [71 J-P. Sung and D.W. Se&r, 3868. 181 D.E. Pritchard, N. Smith, RD. Driver and T.A. Brunner. J. Chem. Phys 70 (1979) 2115. f91 T.A. Brunner, N. Smith, A.W. Karp and D.E. Pritchard, J. Chem. Phys 74 (1981) 3324. 1101 K.L. Saenger, N. Smith, S.L. Dexhelmer, C. Engelke and D.E. Pritchard, J. Chem. Phyr 79 (1983) 4076. 1111 K Bergmanu. R Engelhardt, U. Hefter and J. Witt, J. Chem. Phyr 71 (1979) 2726; K Bergmanu and W DemtiUder. Z. Physik 43 (1971) 1; J. Phys B5 (1972) 2098. 1121 W-M_ Jackson and H. Okabe, Adven Photochem. 13 (1985), to be published. [I31 F. ShokooM S. Hay and C. Wittig, Chem. Phys Letters 110 (1984) 1. 1141 M. Sabety-Dzvonik and R Cody, J. Chem. Phys 64 (1976) 4794. M. Heaven, T.A Miller and V-E. Bondybey, Chem. [ISI Phys. Letters 84 (1983) 1. [16] I. Nadler, M. Noble, H. ReisIer and C. Wrtt& J. Chem. Phys.. to be published I171 A. Mele and H. Okabe. J. Chem. Phvs Sl(l969) 4798 218j J-1. Steiufeld and W. l&emperer, J. Chem. Phys 42 (1964) 3475; R.B. Kurzel and JJ Steinfeld, J. Chem. Phys 53 (1970) 3293; RB. Kruzel, J-1. Steinfeld, DA Hatzenbuhler and G.E. Leroi J. Chem Phys 55 (1971) 4822. [19] S. Hay, F. Shokoohi, S. CaJlister and C Wittig, unpublished results. 1201 D.L. Thompson, J. Chem. Phys 77 (1982) 1286. 1211 L.H. Beard and D-A Micha, J. Chem Phyr 74 (1981) 6700. [ 221 W. J Moore, Physiical chemistry, 4th Ed. (Prentice Hall, EngJewood Cliffs, 1972). [23] P.C Teliinghuisen, J. Teilinghuisen, J.A Coxon. J.E. Velazw and D.W. Setser. J_ Chem Phys 68 (1978) 5187. [24] J.M. Hoffman, G.C. T&one and AK Hays, J. Chem. Phys 64 (1976) 2484. [25] M. Krauss and FH_ Mies Topics in apphed physics, VoL 30. ed_ C.K. Rhodes (Springer, Berlin, 1979); M.C. Lin, MB. Urnstead and N. Djeu, Ane Rev_ Phys Chem. 34 (1983) 557. 1261 C. Ottinger and M. ScbrBder, J. Phys B13 (1980) 4163; S-L. Dexheimer, M. Durand, T_k B~UMIX and D.E. Pritchard, J. Chem. Phys 76 (1962) 3996. (271 L NadIer, K Reisler. M. Noble and C. Wittig, Chem. Phyr Letters 108 (1984) 115. 1281 L. Lam, C. Dugan and C.M. Sadowski, J. Chem. Phys. 69 (1978) 2877. 1:
CHEMICAL
PHYSICS
12 July 198.5
LETTERS
COLLISIONAL METASTABILITY OF HIGH ROTATIONAL STATES OF CN(X ‘2 +, d’ = 0) * S. HAY, F. SHOKOOHI,
S. CALLISTER
Chemrsrry
of Southern
Depcutment.
Uniwrsr@
and C. WI-lTIG
Cahfotnicq
LDs AngeL,
CA 90089-0484.
USA
Rcceivcd 14 February 1985; in final rorrn 10 May 1985
CN(X ‘z +. u”= 0) high rotational stales relax slowly via 300 K collisrons with Ar and Kr_ Relaxation decrease with increasing relation. and the partially relaxed dlstnbutions are bimodal. with low N” thcrrnalkd (300 K), and N”= 80 unrelaxed afier loo0 collisions. Relaxation by N,, CO, and Xe is similar to Ar and Kr, but more efficient. He and NO remove many quanta in a smgle co&ion
1_ Intro4hction It is often assumed that the rotatronal degrees of freedom of small gaseous molecules are efticrently thermalized by collisions with inert species. The spacings between adjacent levels are usually small compared to kT, and it is not uncommon for rotational energy transfer cross sections to exceed those for hard-sphere collisions [ 11. However, this rule of thumb is not universal_ For small moments of inertia and/or large rotational quantum numbers, the spacings between adjacent levels can be larger than kT, and rapidly rotating molecules may appear more isotropic durmg a collision than their slowly rotating counterparts With hydrogen halides, for example, high rotational states can survive many collisions with Ar [2-71, and this propensity can help explain the operating characteristics of hydrogen halide rotational lasers 151. With heavrer nuclei, the same propensity persists, and Pntchard et al [8-101 and Bergmann et al. [ll] have combined careful deactivation measurements with theoretical models, thereby providing quantitative bases for predicting rotational exchange for dlatomlcs. By using photodissociation to produce rotationally excited diatomics, ground electronic-state species can be prepared in rather high rotational states. For * Research
6
supponed by the Nahonal Scicna Foundation
example, the 193 run photodissociation of BrCN produces CN with little electronic or vibrational excitation (~1% in the A 211 state [12], 66% wrth U” > 1) [12], and withN” up to 86, corresponding to as much as 14000 cm-l of rotational energy. With room-temperature samples, the spacings between adjacent rotalional states withN” z 80 are comparable to the average translational energy, and based on previous research, one expects these h@ZV” to be metastable with respect to collisions with small spherically symmetric species [13-G] _ In this cornnunication, we report the direct observation of this metastabrlity, and note that with certain partners a large number of collisions are required in order to thermalize the CN rotational degrees of freedom_ For example, the average rotational energy, UZ,,,), of the nascent CN produced by the 193 nm photodissociation of BrCN is 6100 cm-l, and thousands of collisrons are required in order to relax (Er,t) down to -00 cm-I= using Ar or Kr buffers. In addition, the partially relaxed distributions are very bimodal, with the low N” therm&red at 300 K, and the lughest N” retaining their nascent distribution.
2. Experimental The experimental arrangement has been described in detail previously [ 131. Briefly, 193 run excimer la0 009-2614/85/S (North-Holland
03.30 0 Elsevier Science Publishers B.V. Physics Publishing Division)
Volume 128, number 1
CHEMICAL
PHYSICS LETIJZRS
ser photolysis of BrCN is followed by laser induced fluorescence (LIF) detection of CN (O-14 cm-l resolution), and signals are digitized and averaged until suitabIe Sfnr is achieved_ A narrow bandpass interference filter centered at 388.5 nm (20 nm fwhm) is-used to lsolate the B 2Z+-X 2Z+ emission. Relative populations are obtained from the spectra following careful procedures which have been described in detail previously [ 161. For example, when the spin-rotation components are not fully resolved, the line intensities are taken from the areas of the absorption lines rather than the peak heights. Ln cases of overlapped absorption lines (e-g , the b~dhead region) or perturbed rotational levels, the relative populations are obtained by fitting the overlapped spectra and/or extrapoIation. BrCN (Aldrich, 97%) is purified by vacuum distillation prior to experiments, NO (Matheson, >99%) is subJetted to repeated trap-to-trap distillations, He, AI, P-branch
12 July 1985
and Kr [Spectra Gases, ultrahigh purity), Xe (Spectra Gases, >!39.99%), N2 (M-G. Industries, research grade, 99.9995%) and CO (Matheson, x!? 99%) are used without further purificatron.
3. Results The spectra shown . I fig 1 reflect the metastability of the high N” with respect to colhsional deactivation by Ar The upper trace 1s for nascent CN, and nnmediateiy below it II; the spectrum of a 300 K sarnpie_ Notice the prom~ent Doppler broadening in the spectrum of nascent CN, and the unequal populations III the spur-rotatron doublets Trace (C) shows the spectrum obtained with 100 Torr of Ar and a 300 ns delay between the photolysis and probe lasers, the high N” are split by spin-rotation interactron and per-
W-
Fig. 1. LIFVspectra of CN(X2Z+, u” = 0). following the 193 nm photolyds of BrCN. In {A), the sample COC~GIIS20 mTon BICN and the delay between the photolysis and analysis lasers in 300 ns The w-m displays nascent CN rotational excitation; the mme spectrum is obtained.with lower BICN pressures and skirter delays Zn (B), 10 aTon BICN, 100 Ton He, and a 500 ns delay provide a completely th ‘- d .mmple (300 x;) for aunpmn purposea Trace (c) (10 mT’o= BrCN, 100 TOH Ax, 300 ns delay) showS pa&al relaxatio& with low IV” thermaked at 300 K Trace (33 isobtained from (C) by removing the low N” and expanding the vertical axis, to aid identification of the highs T-kuuzh transitions (+ denotes the perhubedN” = 60 level).
7
Volume i 18.
number
1
4
ElrCN-!==--+
12 July 1985
PHYSICS LE-ITERS
CHEMICAL
Br + CN(X*E+. vu. N’)
d
t
amA
A” A A 0
IO Torr
Ar
*
100 Torr
Ar
‘3
“934
0 1
0
‘3
bdra
noscenl
1
10
,
I 20
5
I
I
30 CN tX=E+, v-=0,
I
40 N’)
I
I
50
ROTATIONAL
I
I
60
I
I
70
1
I
80
LEVELS
2. Rotational populatinn distributionsfor nascent conditions, as well as 10 and 100 Torr 01A.r (300 ns delay). The b&nodal nature of the pattially relaxed distrz%utionsis evident; notice that the extent of relaxationdecreasesmonotonically with mcrea+ Fjg.
ingN”_
-
sist even though many (=300) collisions have occurred (13]_ Since the high N” are split and spread over more levels than the lower N”, they contain a larger fraction of the population than one might infer casually from the rclatlve peak heights. For example, 60% of the population have N” 2 40 with 100 Torr Ar and 300 ns delay, corresponding to Er,,~ = 4300 cm-l_ Trace (D) is obtained from the one above it by removmg the 300 K component and expanding the scale, in order to make identifications easier. The small peaks on the short-waveleng~ side of N“ = 62 arc due to u“ = 0 R-branch and u” = 1 Pbranch transitions_ We estimate that nascent CN vibrational excitation LSweI1 withm the limits reparted previously [ 12). As in the work of Jackson et al. [ 121, we also observe that U” = 1 populations increase dunng the mitial transient phase, in which nascent translational excitation is degraded_ The extent of increase depends on the collision partner, with the heavy rare gases (& and Kr) leading to increases of the u” = 1 population of as much as a factor of 3. The rotational d~trlbutions associated with spectra such as the one shown in fig. lc are very b&nodal, and exampies are shown in fig. 2. The lower states are 8
well characterized by a 300 K B~ltv~la~ dlstnbution, and the high states show the effect of the collisional transfer rates decreasing with increasmgN”_ The distributions in fig. 2 show the collislonal metastability of the highest rotational states detected ior the present experiments. Although we presently monitor levels withN” 4 80, the VW photolysis of ICN is known to producefV”’ up to 107 [17f, having -21100 cm-l of rotatlonal energy and spacings between adjacent levels of WOO cm- 1 _The physics of such species has never been studied experimentally, and we note that by optically pumping to the B 2Z* state, isolated highN can be studied as per the methods pioneered by Steinfeld et al. [18], Pritchard et al. [S-10], and Bergmann et al. [ 1 l] in cases where rotational relaxation is more efficient in the B 2E? state than the X 2Z+ state. The distributions shown in fig. 2 indicate the main features of the relaxation of rotationally excited CN via cohzons with Ax. Orice below N” EYU 40, relaxation 1s sufficiently rapid that species are therrnaiized at 300 K on the time &ale of the measurements, Above N” = 40, however, relaxation is slow, with the highest N” showing the feast amount of relaxation,IL? 2 67
Volume 11.S, number 1
CHEMICAL
is essentially unrelaxed. Considerable relaxation of N” = 57-67 occurs during the first few collisions as demonstrated by pronounced differences between the nascent distribution and that obtamed with 1 Torr Ar [ 19]_ The high nascent velocities from BrCN photo. dissociation result in relatively efficient rotational relaxation dunng the initial transient phase [20,21], in which the hyperthermal velocities are degraded to 300 K In the present experiments, the maximum pressure was limited to 100 Torr by the flow arrangement, as well as the need to insure that CN(B 2Z*, u’ = 0) rotational levels are not appreciably quenched with N” dependent cross sections. Under these conditions, relaxation 0fN” = 80 is quite small (i.e. there is no measurable relaxation with -lo3 colbsions), and 2104 collisions may be requked in order to relax these levels efficiently. In the case of k, relaxation is slightly less efficient than AI, as can be seen in table 1. However, the qualitative behavior 1s almost identical to that shown in fig. 2, and spectra taken with 100 TOIT of Kr and several s delay show prominent highIV” populations. N2 is more efficient than Ar, and as with Kr, the qualitative behavror is almost identical to that shown in fig. 2. CO and Xe are more efficient than N2, and again the bimodal nature of the partially relaxed distribution is
BrcN 0
.
12 July 1985
PHYSICS LEmERS
---==G-
Table 1 Relaxation of the averageCN rotational energy, arot>, by
different co&sion partners The number of collisions,R=, from data tabulated in reE [22]
Collision partner
narcent 3 IG Ar
Pressure
Delay
(T-1
W
0.02 100 1.0 10
300 600 300 300
30 100
300
35 11
N2
Xe He
1.0
10 30 100 NO
0.5 1.0
300
300 300 300 300 500 500 300 300
(Elot) (cm-l)
nC
6100 4200 5300 4650 4400 4300
700 b) 37
2000
150
2850
110
370
b)
42
4900 1900 210 200
60 300 1000
3400 2400
b, e) b, c)
a) The spectrumdoes not changenotmzably at lower pxe? sums and/or shorter delays. b) At these low pressures,degradationof the na.5centtransliltional energymakes it impotibk to estimate the number of colkions. a The reactivenature of the CM/NO system makes hardsphere collision moss sections inappropriate.
Br + CN(X’E+:+.
0
100 Torr
He
0
!O Ton-
me
*
f Torr
He
CN(X'LC+. v"=O, N-1
is
caldated
ROTATIONAL
v”. N")
LEVELS
Fig 3.Rotationalpopukition distributionsfor 1, 10, and 100 Ton of He (300 ns deWI.
9
Volume 118. number 1
CHIZIICAL
PHYSICS
12 July 1985
LETTERS
conserved. In the case of He (fig. 3) there is much less bimodal character in the distributions than wrth Ar, Kr, N,, CO or Xe It appears that the high velocity and small size of He allow rt to penetrate the potenteal of rotating CN and provide enough torque to remove a number of ro tational quanta in a single en-
tional states, removing multiple quantain a single collision [9,26], and this can account for the partially relaxed distributrons having httle bimodal character_ Smce He is small and light, it can get close to rotating CN, thus providing the torque which is required for rotational exchange_ Similarly, the large nascent CN
counter_ The high quenching efficiency of Xe relative to Ar and Kr can be attributed to the chemistry of this specres. The ground potential surfaces of XeF and XeCl
translational excitahon can facilitate energy transfer with heavy species such as Ar, durmg the initial transient phase in which translations are thermahzed. This can be seen by noting that rotations are relaxed more efficiently prior to translational thermalization than
are bound by 1065 cm-l [23] and 255 cm-l [24] respectively, and based on CN’s electronegativity, we expect the XeCN bond strength to lie between these values. Thus, the mteraction energy is much stronger than one would infer from the dipole-induced-dipole
afterwards. The relaxation of rotationally excited CN by collisions with NO is very efficient, as shown in fig. 4. Many quanta .are removed in just a single collision,
contribution, and this can enhance the energy transfer cross sections. With ArF and KrF, the ground potential surfaces are essentially repulsive [25] and we anticipate no sign&ant attraction in either the Ar/CN or
and the spur-rotation more rapidly than the smce NCNO IS bound CN + NO [27], and a
Kr/CN systems. Quahtatively, collisional relaxation by the rare gases He, Ar, and Kr follows the trends reported by Brunner et al. [9] in theu study of rotational relaxation of Na,. It is known that He can efficiently relax high rota-
may form an NCNO* intermediate. In dissociating, NCNO* wdl apportion the energy m excess of reaction threshold statistically, thereby causing efficient relaxation of the initial CN R,T energy and equalization of the spin-rotation states [19]. It IS known that
BrCN v
.
*“A
_
193nm
Br + CN (XCEf,
0.5 Tom 1
Torr
states are equilibrated even rotations. This is not surptising, by 49 kcal mol-1 relative to large fraction of the collisions
Y”,
N=)
NO 43
NO
P
P
P
* n
0
20 30 CN CX’E+, v-=0,
40 N-1
ROTATIONAL
50
60
70
El0
LEVELS
Fig. 4. Rotational population distilbutions for nascent conditions, as well as 0.5 and 1.0 Torr of NO (300 us delay). Even w&h 0.5 Tom NO, many rotational quanta have been relaxed Notice the similarity between the cases of 1 Torr NO and 10 Torr He (t-&G3). 10
Volume 118. number 1
CHEMICAL
PHYSICS
CN reacts with NO at 300 K forming N2 + CO (1.2 X lo-l3 cm3 molecule-1 s-l) [28], and it is possible that some species are removed by reaction rather than melastic scattering. However, in comparing the signal intensities of thermallzed CN samples for the cases of He and NO, we note that the hyperthermal nascent CN is not consumed efficiently by NO. Perhaps the bimolecular reaction: CN + NO + N, + CO transpires via the NCNO* complex U-Icompetition with dissociation back to CN + NO reagents. In most of the experiments listed above, the BrCN pressure was lo-20 ml‘orr With added gas pressures that significantly exceed that of BrCN, the translational degrees of freedom are thermaltied rapidly, so that collisions occur at a 300 K rate. Thus, with the BrCN pressure constant, we observe changes due to the added gas, under conditions in wluch the CNBrCN collisions occur at a 300 K rate This insures that effects are due to the additive, and not an alteration of collisions between translatronally hot CN and the BrCN substrate There are several reports in the literature of 121% rotational states being relaxed inefficiently via collisions with rare gas atoms. The present results confYm
the established qualitative trends [6,14,15,27] and mdicate that such metastability might be quite pronounced under certain circumstances. For example, with 100 Torr of Kr buffer and delays of 2 ps, there is little relaxatron of levels withN” = 80. These CN molecules have experienced >1 O3 collisions with little loss of energy, and in order for them to become “therrnalized” at 300 K. it will require passage from N” 2; 80 to N” < 20, and this wrll probably require 21 O4 collisions. With even higher N” (e-g_, N” 5 107 are prepared by the VUV photodissociation of ICN [ 171).and/or lower ambient temperatures, an even greater degree of metastability is anticipated_ Thus, in interstellar media, where collisions are infrequent and VUV photolysis IS common, such metastability can play a significant role m chemicaI as welI as radiative transport processes. References J-T_ Yardley, Introducbon to molecular energy transfer (Academic Press, New York, 1980). [ 21 H.K. Haugen, W-H. Pence and S.R. Leone. J. Chem Phys 80 (1984) 1839. [ 31 J.C. Polanyi and K.B. Woodali. J. Chem. Phya 56 (1972) 1563.
[l]
LETTERS
12 July 1985
[41 J.A. Barnes M. Keil, RE. Kutina and J.C PolanyI, J_ Chem_ Phys 72 (1980) 6306. 13 0-D. Krogh and G.C. Punentel, J_ Chem Phyr 67 (1977) 2993. 161 D.J. Bogan, D.W. Sctrer and J.P. Sung, J. Phys Chem. 81(197T) 888. J. Chem. Phys 69 (1978) [71 J-P. Sung and D.W. Se&r, 3868. 181 D.E. Pritchard, N. Smith, RD. Driver and T.A. Brunner. J. Chem. Phys 70 (1979) 2115. f91 T.A. Brunner, N. Smith, A.W. Karp and D.E. Pritchard, J. Chem. Phys 74 (1981) 3324. 1101 K.L. Saenger, N. Smith, S.L. Dexhelmer, C. Engelke and D.E. Pritchard, J. Chem. Phyr 79 (1983) 4076. 1111 K Bergmanu. R Engelhardt, U. Hefter and J. Witt, J. Chem. Phyr 71 (1979) 2726; K Bergmanu and W DemtiUder. Z. Physik 43 (1971) 1; J. Phys B5 (1972) 2098. 1121 W-M_ Jackson and H. Okabe, Adven Photochem. 13 (1985), to be published. [I31 F. ShokooM S. Hay and C. Wittig, Chem. Phys Letters 110 (1984) 1. 1141 M. Sabety-Dzvonik and R Cody, J. Chem. Phys 64 (1976) 4794. M. Heaven, T.A Miller and V-E. Bondybey, Chem. [ISI Phys. Letters 84 (1983) 1. [16] I. Nadler, M. Noble, H. ReisIer and C. Wrtt& J. Chem. Phys.. to be published I171 A. Mele and H. Okabe. J. Chem. Phvs Sl(l969) 4798 218j J-1. Steiufeld and W. l&emperer, J. Chem. Phys 42 (1964) 3475; R.B. Kurzel and JJ Steinfeld, J. Chem. Phys 53 (1970) 3293; RB. Kruzel, J-1. Steinfeld, DA Hatzenbuhler and G.E. Leroi J. Chem Phys 55 (1971) 4822. [19] S. Hay, F. Shokoohi, S. CaJlister and C Wittig, unpublished results. 1201 D.L. Thompson, J. Chem. Phys 77 (1982) 1286. 1211 L.H. Beard and D-A Micha, J. Chem Phyr 74 (1981) 6700. [ 221 W. J Moore, Physiical chemistry, 4th Ed. (Prentice Hall, EngJewood Cliffs, 1972). [23] P.C Teliinghuisen, J. Teilinghuisen, J.A Coxon. J.E. Velazw and D.W. Setser. J_ Chem Phys 68 (1978) 5187. [24] J.M. Hoffman, G.C. T&one and AK Hays, J. Chem. Phys 64 (1976) 2484. [25] M. Krauss and FH_ Mies Topics in apphed physics, VoL 30. ed_ C.K. Rhodes (Springer, Berlin, 1979); M.C. Lin, MB. Urnstead and N. Djeu, Ane Rev_ Phys Chem. 34 (1983) 557. 1261 C. Ottinger and M. ScbrBder, J. Phys B13 (1980) 4163; S-L. Dexheimer, M. Durand, T_k B~UMIX and D.E. Pritchard, J. Chem. Phys 76 (1962) 3996. (271 L NadIer, K Reisler. M. Noble and C. Wittig, Chem. Phyr Letters 108 (1984) 115. 1281 L. Lam, C. Dugan and C.M. Sadowski, J. Chem. Phys. 69 (1978) 2877. 1: