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Vibrational relaxation of methyl fluoride in the temperature range 300 to 150K
Volume
120, numbcr 6
CHEMICAL
PHYSICS
LETTERS
25
October 1985
VIBRATIONAL RELAXATION OF METHYL FLUORIDE IN THE TEMPERATURE RANGE 300 TO 150 K CT
WICKHAM-JONES.
Rrxened
S L LUNT
’ and
C J S-M. SIMPSON
29 July 1985
Raw con\,ant~ [or the \Ibrauonal dencurn~ron of CH,F CH,F He Ne. Ar Kr. H2 N1 and CO, have been measured
I _ Introduction Detailed studres have been made of energy transfer between the vtbratronal modes of methyl fluoride at room temperature by Flynn and co-workers [ 1,2] They showed that the (W) rate constants connecting the internal vibrational modes are very much faster than those for the (VT) relaxation from the lowest mode All the vrbrational modes can therefore be regarded as coupled on the tune&ale of the vibrational deactrvation of the methyl fluoride This condition applies both to pure methyl fluorrde and to the mixtures employed in tlus research For a consrderable number of molecules the rate constants for W couphng with CH,F are much greater than those for (VT) deactivation of CHSF. Methyl fluorrde, once excited, can thus be used in collisional pumpmg in the literature so far mclude CH3Cl [3], NOz 143, NO, CO [5] and 0, [6], all at room temperature In thus laboratory we have used CH,F as a collisronal pump to study the deactrvation of the 15 /-rrn band of CO2 by a varrety of colhsion partners over the temperature range 300 to 150 K [7] In order to interpret these results rt was necessary to have values for the rate constants for the deactivation of CHjF by the species m the gas mixtures employed. There ’ Prcwn~ _uldrers SERC. Warrrng~on
542
Chrshrre
UK
Darabury
LJborJtoq
Daresbury
I” the ~empernwrs r.mge 300-150 K by the colhs~on pdrlncrs wth n laser-Induced fluora~cncc txhnlqus The use of CH,F as
have been measurements of the rate constants for the deactivation of CHJ F by a variety of collision partners at room temperature [8], but none at lower temperatures- In this paper we report values for (VT) rate constants for the deactivation of CH,F m the temperature range 295 to 150 K for the collisron partners CH,F, He, Ne, Ar, Kr, H, and N, and (W) rate constants for the transfer to CO2
2 Experimental The fluorescence cell and data handhng method have been described recently [9] We pumped the v3 band of CH3F with the P(20) lure of the 9 6 m band (1046 85 cm-l) of a pulsed CO, laser. Fluorescence was observed from the u6 band of CH,F at 1182 cm-l A senes of interference tilters was used to minimrse the scattered laser pulse and pass the fluorescence signal, monitored by an Infrared Associates HgCdTe detector. All gas mixtures were prepared with the following research grade gases- CH,F (99 9%) from Columbia
Orgamc Chemicals, He (99.9950/c), Ne (99 998%), .Q (99 9995%), Kr (99 99%), H2 (99 9995%), CO, (99 999%), N2 (99.994%) and 0, (99 97%) were all obtained from the British Oxygen Corporation The CH3 F was further purified by coolmg the cylmder with sohd CO2 for several hours. This was found to rnfluence the rate constant for CH,F selfdeactiva0 009-2614/85/S (North-Holland
03 30 0 Elsevrer Science Pubhshers Physrcs Pubhshmg Drvrsron)
BV
Volume
120, number 6
CHEMICAL
PHYSICS
LETTERS
25 October
overall decay rate of the fluorescence fluoride m the mixture.
tlon, reducmg it by 30%. Subsequent punfication was found to leave the relaxation rate unaltered As a further precaution a solid COZ/propan-2-ol cooled trap was applied to a finger on the glass bulb containing the gas mixtures Tlus was left in place for 2-3 h before use In the mitral stages of these experunents considerable dtificulty was experienced m producing fluorescence with a smgle-exponential form This was due to the drrect influence of the laser on the detector. The problem was solved by substantially reducmg the Intensity of laser light reaching the detector. llus was aclueved by mounting an mterference falter mslde the detector Under the condltlons of our experiments the energy transfer between the modes of methyl fluoride occurs too rapldly to be followed with our detection system which has an overall response time of a few microseconds_ For this reason we were unable to observe the rise of the fluorescence, but could monitor its decay. The composltlons of the gas mixtures were chosen so that the process being investigated dominated the
from the methyi
3. Results 3 1. Rate constatits for deactivahon by methyl fluonde and the inert gases These rate constants have been determined using the gas mixtures given in table 1 To measure the rate constant for the self-deactivation of CH, F It was necessary to use a murture contammg some mert gas, in this case Ar This 1s to prevent the vibrationally excited CH3 F from diffusing out of the field of view of the detector during the observation tune, and to substantlally reduce the corrections which have to be apphed to the rate constants due to thermal heating effects [2] _Tlus presupposes that one knows that the rate constant for deactivation by Ar Thus three mixtures were used and the relaxation time of each was plotted as a function of mixture composition The self-deactivation
Table 1 hllrrure cornposItions CH3 H/He
CH3 F/Ne
CH31-
He
(1) (7-I
6 017% 6 12670
93 988%
(3)
4 188%
93.874% 30 231%
Ar
CH3 F (1)
(1) (2) (3)
Ar
6 462%
93 538%
(1)
1 621% 0 606%
98 379% 99 394%
(2)
CH3 I-
Kr
1 625% 0 788%
98 3755 99.212%
CH3 l-:H 2
CH3 F
NZ
1 296%
98 704%
CH3F (1)
CH3 F/CO2
(1)
98 75%
CH3 T/l-h
CHa F/N,
(1)
1 25%
Ne
65.581%
CH3 FlAr CH3T
1985
CH3F
HZ
CO2
Ar
1 725%
0 171%
4 305%
93 799%
4 436%
Hz
0 894%
Ar 94 67%
Volume
120, number
Table 2 Rate constants Temp
6
CHEMICAL
of CH3H’
for rhc dcactwation
295
1 8 (-14)[0
270 250 220 190 170 150
295 270 250
220 190 170 150
I] b)
‘) Rate constant b)ForA(B)[C]
25 October
1985
1 1 9 6 5
4.5
Kr
5 0 (-16)[0 2] 4 1 (-16)[0 I] 3 4 (-16)[0.2]
AI
Ne
I 95(-14)[0
1.4 (-14)[0 I] l-20(-14)[0 OS] 1 lO(-14)[0 OS] 1 lO(-14)[0 OS] l.lO(--14)[0 OS] 1 lO(-14)[0.101
2 6 (-16)[0 11 2 05(-16)[0 051 1 80(-16)IO.15] 1 60(--16)[0.15]
LETTERS
by Ma) HC
CH3 F
(I()
PHYSICS
OS]
60(-14)[0 OS] 30(-14)[0 OS] 5 (-14)[0 41 9 (-15)[0.3] 6 (--15)[0.2] (-15)[0.2]
1 10(-lS)[O
8 6 4 3 2
6.4 (-16)[0 41 4.9 (-16)(0 41 3 90(-16)[0 IS] 2.75(-16)[0.15] 2 OO(-16)[0.10] l-65(--16)[0 lo]
OS]
1 (-16)[0.2] 35(-16)[0 151 40(-16)[0.100] 05(--16)[0 IO] 45(-16)[0 lo]
2.05(-16)[0.10]
140(--16)[0.15]
N2
H2
co2
1 15(-15)[0 051 8 5 (-16)(0 41 6 9 (-16)[0.3]
6 0 (-13)[0.2] 4 9 (-13)[0.21 4 2 (-13)[0 21
1.15(-l 3110 051 1 lO(-13)[0.10] l-10(-13)[0 051
5 0 (-16)[0 3.7 (-16)[0
21 21
3 1 (-16)(0
3]
2 6 (--16)[ 0.51
3 30(-13)[0 IO] 2 60(-l 3)[ 0 051 2 25(-13)[0 051 2 OO(-13)[0 051
1 lO(-13)[0 l-10(-13)[0 1 lO(-13)[0 1 15<-13)[0
OS] OS] 051 201
umts ax cm3 molecule-’ 5-l read (A -+ C) X lo8
for 95% confidence
lures
and AI rate constants were obtamed by fitting a straight line by least squares In the mixture
with
most
CH3F
the contnbutlon
by the CH,F amounted to 70% at 295 K and 85% at 150 K The rate constants for He were measured usmg three mixtures and those for Kr usmg two. TIIIS was done to check the consistency of the rate constanrs. In all cases the results from each mixture agreed withm experunental error The rate constants for the self-deactivation of methyl fluonde and Its deactwatron by He, Ne, Ar and Kr have been measured at 295 K by Flynn and co-lboorkers [S] They reported a rate constant for the selfdeactlvallon which IS m excellent agreement with our result at 295 K. Their values of the rate constants for deactivation by the Inert gases at 295 K are between 20% and 100% greater than those reported here The results are presented m table 2 and figs 1,2 and 3. to the relaxation
3 2 Rate constants for deactivation
by Hz, N, md
CO2 The vlbratlonal can take 544
place
deactivation
by a (VV)
of CH,F
and a (VT)
by a molecule
route,
F
,,.,5. 200
250
T/K
300
Fg 1. Rate constants forthe deactivation of CH3F as a function of temperature for the collision partners CH3F q, He
As Hz o and CO2 v,
this research, results for the deactivation
ofCH3F by CHsF n, He A and Ha a. ref_ [S].
Volume 120. number 6
CHEhlICAL PHYSICS LEITERS
Fig 2 As fi 1 but for the coll&on partners Ne 0 and Kr P, this research, Ne 0 and Kr v, reP 181 CH3F*
+ M = CH3F + M*
(W)
CH,F*
+M=CH3F+1M
(VT)
3
For H, the energy of the lowest excited level IS 3110 cm-l above that of CH,F and of N2 1s 1082 cm-l above that of CH3F Hence for these collision partners the Bolzman factor governmg the energy
25 October 1985
distnbution between these partners at equlllbnum greatly favours the CH,F and the measured rate constants appertain to its (VT) deactivation. Weitz and Flynn reported a rate constant at 295 K [83 for deactivation by H2 which is 30% lower than us and by N2 which 1s within our experunental error of 5% These results are presented in table 2 and figs. 1 and 3 For CO2 as a collislonal partner there are many (VV) processes which are close to resonance and the rate of(W) transfer to CO, is greater than that of the WT) deactivation of CH3F by CO, In order to ma1.e a direct measurement of the forward (W) rate constant from CHSF to CO2 we work under condltions in which the back reaction is neghable. This is achieved by adding hydrogen to the mixture as this molecule deactivates the bending mode of CO2 extremely rapidly At 295 K the rate constant IS 5.0 X lo-l2 cm3 molecule-1 s-l [lo] compared with 6.0 X lo-l3 cm3 molecule-l s-l for the deactivation of methyl fluoride by hydrogen. The results obtained for the (W) transfer from CH,F to CO, are given in table 2 and fig 1
4. Discusion The rate constants for the self-relaxation of CH3F are about three tunes greater than those for the selfrelaxation of the bend-stretch manifold of CO1 111 the temperature range 300 to 150 K despite the fact that tne lowest vIbratIonal frequencies are 1049 and 667 cm-l, respectively This must reflect the rmportance of the large dipole moment of CH,F on the rate of vibratlonal energy transfer and probably the ability of CH,F to take up energy m rotation.
The rate constants for the deactlvatlor of these two molecules by the inert gases are closer Those for the deactivation of CO2 fall with temperature about twice
Fig 3. AS fg. 1 but for the collison partners Nz fi and
tld.5 research,Nz ~andArm,ref
[S].
as much as do those for the deactivation of CH,F It is interesting that the heavier gases Kr and AI deactivate CHJF about twce as fast as CO, whereas for the lighter collision partners Ne and He the positlon is reversed At 295 K the rate constant for the deactivation of CH3F by Ar is greater than that by Kr, but at 150 K the rate constants are identical within the experunental error - if anything those for Kr are slightly h&er This may be due to the greater unportance of the attractive forces with Kr at low temperatures_ In this case CH3F behaves similarly to CC4 for which 545
Volume
120, number 6
CHEMICAL
PHYSICS
the rate constants for deactlvatron by AI are greater than those for deactivation by Kr at 295 K, but Identlcal wlthin experimental error at 170 K [1 I]. Nitrogen behaves hke He and Ne m that it deactrvates CO2 faster than CH3F with rate constants which are sunllar to those of Ne Hydrogen behaves very drfferently as a colhsion partner for these two cases At 295 K H2 deactrvates CO, eight tunes faster than CH,F In the case of CO2 the rate constant Increases with falling temperature whereas with CH3F it falls These differences can be accounted for by the fact that with CO2 there is an efficient near-resonant VR process [lo], but the frequencies of the lower vibrational modes of CH3F preclude a near-resonant energy transfer process takmg place at low temperatures The rate constant for (VV) transfer between CH,F and CO2 at 295 K IS an order of magnitude slower than the transfer between CH3F and N20 [ 121 In the latter case the transfer is into the (1000) level wluch IS at a similar energy to the [ 1 O”O, 02°0]n level m CO,. However it IS hkely that transfer mto the IR inactive band of CO, IS much less efficient than mto the IR active band of N,O even though the energy mismatches of the two processes are surular. We have not attempted to calculate rate constants for these processes usrng the theory of vibrational to translational energy transfer developed by Schwartz, Slawsky and Herzfeld [ 131. This theory has been shown to be seriously inaccurate [ 14]- More sophlstlcated theories exist and are particularly applicable to the cases of diatom/atom [ 14,151, diatom/diatom [16,17] and tnatom/atom [IS] scattering These have been extended to the symmetric top/atom case by Clary [ 191 This theory is at present bemg applied to CH, F and our dara are essential for a comparison because it covers a range of temperatures
LEl-TERS
Acknowledgement We should like to thank the US AU Force Geophysics Laboratory for support for this work, and the Gassiot Grants Committee of the Meteorologcal Office for a grant for supor for CTWJ
References r11
I41 151
R S Sheorey and G Flynn, J. Chem Phys 72 (1980) 1175 J-T Yardley, Introduction to molecule energy transfer (Academx Press, New York, 1980) pp 163-168 S hi Lee and A M Ronn, Chem. Phys Letters 24 (1974) 535. S hf. Lee and AM. Ronn, Spectry Letters 8 (1975) 915 S-M Lee and AM. Ronn. Chem Phys Letters 26 (1974)
I61
491_ J M Preses. G-W
!Zl 131
(71 181 191 [lOI [Ill
IllI 1131 [I41
546
1985
low temperatures. However with a careful choice of mixture compovtlon this is not a severe limrtatron and we have been able to use CH3 F successfully as a collisional pump of the 15 flrn band of CO, and measure rate constants for the deactlvatron of this level of CO, by other colhslon partners [7] _
5 Conclusion The rate constants for(W) transfer between CH3F and CO, are h&er than those for the self-deactlvatlon of CH,F. The CO, acts preferentially to transfer energy mto Its vibrational modes rather than deactivate the CH,F. It is this fact which makes CH-,F a good colhs~on partner for the excitation of the 15 PTI band of CO,. Its disadvantage 1s that It dwctlvates the 15 urn band of CO2 quite efficiently especially at
25 October
Flynn and E Welta. J_ Chem. Phyr,
69 (1978) 2782. S L. Lunt, C-T. Wrckham-Jones and CJ.S M Sunpson. Chem Phyr Letters 115 (1985) 60 E Wertz and G Flynn, J. Chcm. Phys. 58 (1973) 2679 E A Gregory, M M Maria& R M Saddles. CT. WickharnJones and C J S M. Sunpson, J. Chem Phys 78 (1983) 3881. D C Allen, T. Scragg and C-J-S M Simpson, Chem Phyr 51(1980) 279 EA Gregory, M R Buckingham. D 2 Clayton, F J Wolfendcn and C J S M Sunpson, Chem Phys. Letters 104 (1984) 393. R K Huddleston
and E Weitz, J. Chem. Phyr 74 (1981)
2879 R N. Schwartz, Z 1. Slawsky and K F Herrfeld, J Chem Phyr 20 (1952) 1591 M-hi Mar&q. E-A. Gregory, C T_ Wlckham-Joncq DJ Cartwnght and C J.S.M. Simpson, Chem Phys 75
(1983) 347 1151 R J Price. D C. Ckry and G D Bilhng, Chcm Phys. Letters 101 (1983) 269. [ 161 M M Maricq, EA Gregory and C-J S M Sunpson, (Them Phyr 43 (1985) 43 [ 171 Z S&5. R Schinke and G-H-F. Drercksen. J. Chem. Phys 82 (1985) 236 [la] DC Clary, J Chem Phys 75 (1981) 209 [:9] D-C. Clary. J. Chem. Phyr I31 (1984) 4466.
120, numbcr 6
CHEMICAL
PHYSICS
LETTERS
25
October 1985
VIBRATIONAL RELAXATION OF METHYL FLUORIDE IN THE TEMPERATURE RANGE 300 TO 150 K CT
WICKHAM-JONES.
Rrxened
S L LUNT
’ and
C J S-M. SIMPSON
29 July 1985
Raw con\,ant~ [or the \Ibrauonal dencurn~ron of CH,F CH,F He Ne. Ar Kr. H2 N1 and CO, have been measured
I _ Introduction Detailed studres have been made of energy transfer between the vtbratronal modes of methyl fluoride at room temperature by Flynn and co-workers [ 1,2] They showed that the (W) rate constants connecting the internal vibrational modes are very much faster than those for the (VT) relaxation from the lowest mode All the vrbrational modes can therefore be regarded as coupled on the tune&ale of the vibrational deactrvation of the methyl fluoride This condition applies both to pure methyl fluorrde and to the mixtures employed in tlus research For a consrderable number of molecules the rate constants for W couphng with CH,F are much greater than those for (VT) deactivation of CHSF. Methyl fluorrde, once excited, can thus be used in collisional pumpmg in the literature so far mclude CH3Cl [3], NOz 143, NO, CO [5] and 0, [6], all at room temperature In thus laboratory we have used CH,F as a collisronal pump to study the deactrvation of the 15 /-rrn band of CO2 by a varrety of colhsion partners over the temperature range 300 to 150 K [7] In order to interpret these results rt was necessary to have values for the rate constants for the deactivation of CHjF by the species m the gas mixtures employed. There ’ Prcwn~ _uldrers SERC. Warrrng~on
542
Chrshrre
UK
Darabury
LJborJtoq
Daresbury
I” the ~empernwrs r.mge 300-150 K by the colhs~on pdrlncrs wth n laser-Induced fluora~cncc txhnlqus The use of CH,F as
have been measurements of the rate constants for the deactivation of CHJ F by a variety of collision partners at room temperature [8], but none at lower temperatures- In this paper we report values for (VT) rate constants for the deactivation of CH,F m the temperature range 295 to 150 K for the collisron partners CH,F, He, Ne, Ar, Kr, H, and N, and (W) rate constants for the transfer to CO2
2 Experimental The fluorescence cell and data handhng method have been described recently [9] We pumped the v3 band of CH3F with the P(20) lure of the 9 6 m band (1046 85 cm-l) of a pulsed CO, laser. Fluorescence was observed from the u6 band of CH,F at 1182 cm-l A senes of interference tilters was used to minimrse the scattered laser pulse and pass the fluorescence signal, monitored by an Infrared Associates HgCdTe detector. All gas mixtures were prepared with the following research grade gases- CH,F (99 9%) from Columbia
Orgamc Chemicals, He (99.9950/c), Ne (99 998%), .Q (99 9995%), Kr (99 99%), H2 (99 9995%), CO, (99 999%), N2 (99.994%) and 0, (99 97%) were all obtained from the British Oxygen Corporation The CH3 F was further purified by coolmg the cylmder with sohd CO2 for several hours. This was found to rnfluence the rate constant for CH,F selfdeactiva0 009-2614/85/S (North-Holland
03 30 0 Elsevrer Science Pubhshers Physrcs Pubhshmg Drvrsron)
BV
Volume
120, number 6
CHEMICAL
PHYSICS
LETTERS
25 October
overall decay rate of the fluorescence fluoride m the mixture.
tlon, reducmg it by 30%. Subsequent punfication was found to leave the relaxation rate unaltered As a further precaution a solid COZ/propan-2-ol cooled trap was applied to a finger on the glass bulb containing the gas mixtures Tlus was left in place for 2-3 h before use In the mitral stages of these experunents considerable dtificulty was experienced m producing fluorescence with a smgle-exponential form This was due to the drrect influence of the laser on the detector. The problem was solved by substantially reducmg the Intensity of laser light reaching the detector. llus was aclueved by mounting an mterference falter mslde the detector Under the condltlons of our experiments the energy transfer between the modes of methyl fluoride occurs too rapldly to be followed with our detection system which has an overall response time of a few microseconds_ For this reason we were unable to observe the rise of the fluorescence, but could monitor its decay. The composltlons of the gas mixtures were chosen so that the process being investigated dominated the
from the methyi
3. Results 3 1. Rate constatits for deactivahon by methyl fluonde and the inert gases These rate constants have been determined using the gas mixtures given in table 1 To measure the rate constant for the self-deactivation of CH, F It was necessary to use a murture contammg some mert gas, in this case Ar This 1s to prevent the vibrationally excited CH3 F from diffusing out of the field of view of the detector during the observation tune, and to substantlally reduce the corrections which have to be apphed to the rate constants due to thermal heating effects [2] _Tlus presupposes that one knows that the rate constant for deactivation by Ar Thus three mixtures were used and the relaxation time of each was plotted as a function of mixture composition The self-deactivation
Table 1 hllrrure cornposItions CH3 H/He
CH3 F/Ne
CH31-
He
(1) (7-I
6 017% 6 12670
93 988%
(3)
4 188%
93.874% 30 231%
Ar
CH3 F (1)
(1) (2) (3)
Ar
6 462%
93 538%
(1)
1 621% 0 606%
98 379% 99 394%
(2)
CH3 I-
Kr
1 625% 0 788%
98 3755 99.212%
CH3 l-:H 2
CH3 F
NZ
1 296%
98 704%
CH3F (1)
CH3 F/CO2
(1)
98 75%
CH3 T/l-h
CHa F/N,
(1)
1 25%
Ne
65.581%
CH3 FlAr CH3T
1985
CH3F
HZ
CO2
Ar
1 725%
0 171%
4 305%
93 799%
4 436%
Hz
0 894%
Ar 94 67%
Volume
120, number
Table 2 Rate constants Temp
6
CHEMICAL
of CH3H’
for rhc dcactwation
295
1 8 (-14)[0
270 250 220 190 170 150
295 270 250
220 190 170 150
I] b)
‘) Rate constant b)ForA(B)[C]
25 October
1985
1 1 9 6 5
4.5
Kr
5 0 (-16)[0 2] 4 1 (-16)[0 I] 3 4 (-16)[0.2]
AI
Ne
I 95(-14)[0
1.4 (-14)[0 I] l-20(-14)[0 OS] 1 lO(-14)[0 OS] 1 lO(-14)[0 OS] l.lO(--14)[0 OS] 1 lO(-14)[0.101
2 6 (-16)[0 11 2 05(-16)[0 051 1 80(-16)IO.15] 1 60(--16)[0.15]
LETTERS
by Ma) HC
CH3 F
(I()
PHYSICS
OS]
60(-14)[0 OS] 30(-14)[0 OS] 5 (-14)[0 41 9 (-15)[0.3] 6 (--15)[0.2] (-15)[0.2]
1 10(-lS)[O
8 6 4 3 2
6.4 (-16)[0 41 4.9 (-16)(0 41 3 90(-16)[0 IS] 2.75(-16)[0.15] 2 OO(-16)[0.10] l-65(--16)[0 lo]
OS]
1 (-16)[0.2] 35(-16)[0 151 40(-16)[0.100] 05(--16)[0 IO] 45(-16)[0 lo]
2.05(-16)[0.10]
140(--16)[0.15]
N2
H2
co2
1 15(-15)[0 051 8 5 (-16)(0 41 6 9 (-16)[0.3]
6 0 (-13)[0.2] 4 9 (-13)[0.21 4 2 (-13)[0 21
1.15(-l 3110 051 1 lO(-13)[0.10] l-10(-13)[0 051
5 0 (-16)[0 3.7 (-16)[0
21 21
3 1 (-16)(0
3]
2 6 (--16)[ 0.51
3 30(-13)[0 IO] 2 60(-l 3)[ 0 051 2 25(-13)[0 051 2 OO(-13)[0 051
1 lO(-13)[0 l-10(-13)[0 1 lO(-13)[0 1 15<-13)[0
OS] OS] 051 201
umts ax cm3 molecule-’ 5-l read (A -+ C) X lo8
for 95% confidence
lures
and AI rate constants were obtamed by fitting a straight line by least squares In the mixture
with
most
CH3F
the contnbutlon
by the CH,F amounted to 70% at 295 K and 85% at 150 K The rate constants for He were measured usmg three mixtures and those for Kr usmg two. TIIIS was done to check the consistency of the rate constanrs. In all cases the results from each mixture agreed withm experunental error The rate constants for the self-deactivation of methyl fluonde and Its deactwatron by He, Ne, Ar and Kr have been measured at 295 K by Flynn and co-lboorkers [S] They reported a rate constant for the selfdeactlvallon which IS m excellent agreement with our result at 295 K. Their values of the rate constants for deactivation by the Inert gases at 295 K are between 20% and 100% greater than those reported here The results are presented m table 2 and figs 1,2 and 3. to the relaxation
3 2 Rate constants for deactivation
by Hz, N, md
CO2 The vlbratlonal can take 544
place
deactivation
by a (VV)
of CH,F
and a (VT)
by a molecule
route,
F
,,.,5. 200
250
T/K
300
Fg 1. Rate constants forthe deactivation of CH3F as a function of temperature for the collision partners CH3F q, He
As Hz o and CO2 v,
this research, results for the deactivation
ofCH3F by CHsF n, He A and Ha a. ref_ [S].
Volume 120. number 6
CHEhlICAL PHYSICS LEITERS
Fig 2 As fi 1 but for the coll&on partners Ne 0 and Kr P, this research, Ne 0 and Kr v, reP 181 CH3F*
+ M = CH3F + M*
(W)
CH,F*
+M=CH3F+1M
(VT)
3
For H, the energy of the lowest excited level IS 3110 cm-l above that of CH,F and of N2 1s 1082 cm-l above that of CH3F Hence for these collision partners the Bolzman factor governmg the energy
25 October 1985
distnbution between these partners at equlllbnum greatly favours the CH,F and the measured rate constants appertain to its (VT) deactivation. Weitz and Flynn reported a rate constant at 295 K [83 for deactivation by H2 which is 30% lower than us and by N2 which 1s within our experunental error of 5% These results are presented in table 2 and figs. 1 and 3 For CO2 as a collislonal partner there are many (VV) processes which are close to resonance and the rate of(W) transfer to CO, is greater than that of the WT) deactivation of CH3F by CO, In order to ma1.e a direct measurement of the forward (W) rate constant from CHSF to CO2 we work under condltions in which the back reaction is neghable. This is achieved by adding hydrogen to the mixture as this molecule deactivates the bending mode of CO2 extremely rapidly At 295 K the rate constant IS 5.0 X lo-l2 cm3 molecule-1 s-l [lo] compared with 6.0 X lo-l3 cm3 molecule-l s-l for the deactivation of methyl fluoride by hydrogen. The results obtained for the (W) transfer from CH,F to CO, are given in table 2 and fig 1
4. Discusion The rate constants for the self-relaxation of CH3F are about three tunes greater than those for the selfrelaxation of the bend-stretch manifold of CO1 111 the temperature range 300 to 150 K despite the fact that tne lowest vIbratIonal frequencies are 1049 and 667 cm-l, respectively This must reflect the rmportance of the large dipole moment of CH,F on the rate of vibratlonal energy transfer and probably the ability of CH,F to take up energy m rotation.
The rate constants for the deactlvatlor of these two molecules by the inert gases are closer Those for the deactivation of CO2 fall with temperature about twice
Fig 3. AS fg. 1 but for the collison partners Nz fi and
tld.5 research,Nz ~andArm,ref
[S].
as much as do those for the deactivation of CH,F It is interesting that the heavier gases Kr and AI deactivate CHJF about twce as fast as CO, whereas for the lighter collision partners Ne and He the positlon is reversed At 295 K the rate constant for the deactivation of CH3F by Ar is greater than that by Kr, but at 150 K the rate constants are identical within the experunental error - if anything those for Kr are slightly h&er This may be due to the greater unportance of the attractive forces with Kr at low temperatures_ In this case CH3F behaves similarly to CC4 for which 545
Volume
120, number 6
CHEMICAL
PHYSICS
the rate constants for deactlvatron by AI are greater than those for deactivation by Kr at 295 K, but Identlcal wlthin experimental error at 170 K [1 I]. Nitrogen behaves hke He and Ne m that it deactrvates CO2 faster than CH3F with rate constants which are sunllar to those of Ne Hydrogen behaves very drfferently as a colhsion partner for these two cases At 295 K H2 deactrvates CO, eight tunes faster than CH,F In the case of CO2 the rate constant Increases with falling temperature whereas with CH3F it falls These differences can be accounted for by the fact that with CO2 there is an efficient near-resonant VR process [lo], but the frequencies of the lower vibrational modes of CH3F preclude a near-resonant energy transfer process takmg place at low temperatures The rate constant for (VV) transfer between CH,F and CO2 at 295 K IS an order of magnitude slower than the transfer between CH3F and N20 [ 121 In the latter case the transfer is into the (1000) level wluch IS at a similar energy to the [ 1 O”O, 02°0]n level m CO,. However it IS hkely that transfer mto the IR inactive band of CO, IS much less efficient than mto the IR active band of N,O even though the energy mismatches of the two processes are surular. We have not attempted to calculate rate constants for these processes usrng the theory of vibrational to translational energy transfer developed by Schwartz, Slawsky and Herzfeld [ 131. This theory has been shown to be seriously inaccurate [ 14]- More sophlstlcated theories exist and are particularly applicable to the cases of diatom/atom [ 14,151, diatom/diatom [16,17] and tnatom/atom [IS] scattering These have been extended to the symmetric top/atom case by Clary [ 191 This theory is at present bemg applied to CH, F and our dara are essential for a comparison because it covers a range of temperatures
LEl-TERS
Acknowledgement We should like to thank the US AU Force Geophysics Laboratory for support for this work, and the Gassiot Grants Committee of the Meteorologcal Office for a grant for supor for CTWJ
References r11
I41 151
R S Sheorey and G Flynn, J. Chem Phys 72 (1980) 1175 J-T Yardley, Introduction to molecule energy transfer (Academx Press, New York, 1980) pp 163-168 S hi Lee and A M Ronn, Chem. Phys Letters 24 (1974) 535. S hf. Lee and AM. Ronn, Spectry Letters 8 (1975) 915 S-M Lee and AM. Ronn. Chem Phys Letters 26 (1974)
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491_ J M Preses. G-W
!Zl 131
(71 181 191 [lOI [Ill
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546
1985
low temperatures. However with a careful choice of mixture compovtlon this is not a severe limrtatron and we have been able to use CH3 F successfully as a collisional pump of the 15 flrn band of CO, and measure rate constants for the deactlvatron of this level of CO, by other colhslon partners [7] _
5 Conclusion The rate constants for(W) transfer between CH3F and CO, are h&er than those for the self-deactlvatlon of CH,F. The CO, acts preferentially to transfer energy mto Its vibrational modes rather than deactivate the CH,F. It is this fact which makes CH-,F a good colhs~on partner for the excitation of the 15 PTI band of CO,. Its disadvantage 1s that It dwctlvates the 15 urn band of CO2 quite efficiently especially at
25 October
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