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Collision-assisted multiple step excitation of CH3F by irradiation with a TEA CO2 laser
Vohme
84.
number
COLLISION-ASSISTED BY IRRADIATION Hide&i
NAKANE
1 December 1981
Cl-lEMiCAL PIIYSK!S LETTERS
2
MULTTPLE
STEP EXCITATION
WITH A TEA CO,
OF CH3F
LASER *
and Soji TSUCHlYA
Received 5 May 1981; in final form 17 August 1981
By irruditlting a mixture of CH3F highly diluted In Ar with ;I TEA CO2 laser, the ~3 overtone emission rises with a rate much larger than the “up-the-ladder” V-V pumping rate indicating that the CH3F is exited up to the 3~3 level almost instantly. This occurs via multiple step excitation assisted by rotational transitions in collisions.
1. Introduction According to the general scope on the mechanism of the vibrational energy flow in molecular species [ 11, if the species is diluted in 3 monatomic gas such as Ar, the role of coflisions is to enhance intramolecuinr intermade vibration-to-vibration (V-V) energy transfer. Thus, when laser photons are absorbed in one vibrational mode, V-V energy transfer from the excited level of the pumped mode to other nearresonant levels occurs to establish a quasiequilibrium distribution among these levels. This type of energy flow mechanism induced by third-body colhsions is confirmed in laser fluorescence studies, for example, on the CH3F/Ar mixture 123 and on the OCS/inertgas mixtures [3 ] . In this letter, we introduce a different type of collision effect: a collision-assisted multiple step excitation; the 2~3mode of CH3 F in Ar is excited to higkrer levels by jrradiation with a strong CO, laser. This intramolecLlar intramode excitation differs from the “up-the-ladder” intramode V-V energy trarrsfer [1] or the IR multiple photon excitation induced by strong laser irradiation [4].
2. Experimental A TEA CO, Iaser (Lumonics 102-2) beam whose section was reduced to 4 by a condenser lens was introduced into a fluorescence cell. The pulse shape of the laser was composed of a spike of 200 ns fwhm and a tail of I .S MSduration when the laser medium was a He/COZ/NZ (3 : 1 : 0.4) mixture, and the maximum fhence of the Iaser was 1.5 .I cme2 (9.6 ym, P(20)]. The cell contained two parallel mir-
cross
rors 12 cm apart from each other to reflect
the laser
of the beam being ==48 cm. The laser-induced fhrorescence collected by a Wet&type mirror system was detected by an InSb detector grout an appropriate IR fitter or a monochromator. The response time of the detector including a preamplifier was ~2 ps. The signal formed by each laser pulse was digitized, and 1024 or 2048 signals were averaged for noise reduction. The sample 8as of CH,F (PCR, research grade) was employed after repeated solidification, and Ar (9Q_999~ pure) was treated by passing through two successive columns of copper chips at 200°C and of molecular sieve SA cooled to -114S”C. Agiven fraction of CH3F iu Ar was prepared by mixing the respective gases of known pressures in vessels with a calibrated volume ratio.
beam
three times. the path length
* This work has been supported in part by a Grant-in-Aid from the Ministry of Education (No. 311606).
322
0 009-2614/81/0000--0000/S
02.75 0 1981 North-Holland
Volume 84, number 2
CHEMICAL PHYSICS LE-ffERS
3. Results and discussion Figs. la-Id show the observed time histories of the overtone emission of CH~ F irradiated with a TEA CO, laser pulse. In fig. I, the emission data obtained in pure CH, F and CH3 F/Ar mixtures with a CH3 F partial pressure of 60 mTorr are compared with each other. The emission in pure CH,F rises slowly to a rather weak broad peak, while when Ar is added to a pressure around 10 Torr_ the rise of the
emjssion becomes too fast to be followed by the present detector system and a peak appears. Following this peak the emission shows a second slow rise to form a broad peak. This difference between the data in figs. la and 1b couId not be attributed to ;I lower laser energy fuence for irradiating pure CHsF than for the mixture, since the rise of the v3 overtone emission in pure CH, F at a pressure lower than 0.1 Torr is much less dependent on the fluence than in a mixture with Ar [S] . In a mixture co~t~~~ more Ar, the initial peak becomes clearer though the second broad peak is hard to recognize. if the initial rise of the emission
Fig. 1. ~uoresce~~ observed dour a wide-band Biter of the 5 pm region in pun? C&F (a) and CHs/A.r mixtures (b. c, d) and that of the 3 pm region (e) induced by the laser icradiation with an energy ffuegce of 0.4 J cmB2 (a).and 1.2 J cm-’ (b, c, d, e). The partial,pressure of CHSF is 0.06 Torr and those of AI are 0 (a), 7.7 @), 19.3 (c) and 70 Torr (a,@. The relative sensitivity of the detecting system for (a)-(e) &16:2:1:1:0.63.
I Decemberi98L
is defmed approximately by a functional form of 1 -e-1/r, the time constant 7 observed in pure CH,F is close to the value of 6.7 ps which is given by -&e “up-the-ladder” rate constant determined by Sheorey and FIRM [6], while the time cons~ts found in the CH3 F/Ar mixtures are much tess than 6.7 ps. Thus, it is concluded that the excitation of the ~3 mode in a CH,F/Ar mixture must be caused by a mechankm
different
from “up-the-ladder”
V-V
energy transfer and that the excitation is assisted by collisions with Ar. Besides this new excitation mechanism, the ~~p-~e-Iadder’~ V-V
pumping of the p3 mode occurs simultaneously resulting in a slow second rise of the emission. However, this rise is found only in mixtures at a pressure lower than -20 Torr, and as the pressure of Ar increases, the second peak appears closer to the time origin. Since the coexistence of Ar may not affect the ladder climbing rate, some other mechanism in addition to the intramode “up-the-ladder”
V-V
transfer nxxst be taken into
account for a full understanding of the intra- and inter-mode energy flow in a mixture of CH3 F higbly diluted in Ar. In order to test the possibility of direct excitation to the 2~3 level by two-photon absorption of the CO, laser, which was formerly suggested by Kneba and Wolfrum 171, the p3 mode of CH3 F was excited by two types of laser pulses; one was composed of a spike with a tail and the other was only a spike, which was vb&ajned from the laser medium without Nz. When the laser energy ffuences of two pulses were of the same magnitude, the peak power of the pulse with the taii was nearly half the pulse without the tail. Nevertheless, the observed emissions induced in the CH3FjA.r mixture at 70 Torr by both laser puises were the same, indicating that the excitation of the v3 mode of CH,F in Ar is not due to a multiple photon absorption. The observed emissions of the 5 and 3 m regions in the CH,F/Ar mixture of 70 Torr are shown in figs. Id and ie, respectively. The 3 trm emission rises rapidly with a rate limited
by the detector
response,
and reaches a pee which exhibits a fast decay foilowed by a slow one. The~observed rapid rise of the emissions in both the 3 and 5 w regions may evidence a very fast intermode V-V energy transfer between the 3~~ level and, the v4 + neighbqring levels as qlready pointed out by the authors in a previ323
Volume 84. number 2
C~ICAL
PHYSICS
LE-ITEFtS
1 December 1981
Table I Frequencies of paragon-ro~tion
a) The
transitions of the ~3 mode near-resonant with the CO2 Iaser iine at 9.6 Brn P(20)
Vi”viationa1 transition
Rotational transition
Wavenumber (R = 0)
1-O
qQK (11)
1047.122
8.04
qQK (121
1046.853
0.03
~QK (131
1046.560
8.81
qPx (1)
1046.907
1.59
2-l
qR~ (8)
1046.91
1.71
3-z
QRK (20)
I046.975
3.63
1046.618
7.08
4-3 qRK (35) -__-- ---. ..- -_~. wavenumber of the laser iine is 1046.854 cm-’
[ 11)
18). Kneba et al. 191 also found a rapid rise of the emission from the iq , ~4 levels induced by a TEA CO2 laser, and concluded efficient V-V energy transfer between the 3u3 and these levels. However, they speculaIed that the increase of the rise rate of the 3 pm emission by addition of Ar was due to enhancement of intermode V-V energy transfer which leads to the energy flow u3 + v6 -+ “2, vs Y4 besides v3 + 2~3 -+ 3u3 4 vl, “4. -+2Y2,205+v~, Since the Ar pressure in their experiment was insufexcitation
)
I
Non-resonance ldvl (GHz) a)
_
011spaper
ficient, the collision-assisted
(cm-’
mechanism
here may not dominate the intramode or ~te~odc V-V energy transfer by self-collisions of CH, F. proposed
fore, the 45 level wotifd be hard to attain due to the very small
population
in such a high rot~+tiond transition rate is on the order of 107--lo6 s-l Torr-I, an Ar pressure > 10 Torr is sufficient for the rotational transitions within the laser pulse duration to cause multiple step excitation. The time-dependent poptdation distribution in each level of the v3 mode may be discussed on the basis of the v3 overtone emission spectrum observed level. Since the rotational
of a monochromator. Fig. 2a shows the emis4.76 #rn which is interpreted as due to the u = 2 * 0 transition of the v3 overtone. The emission by use
sion at
The frequencies of the ~bration-rotation transitions of *he v3 mode which are near-resonant with the CO, laser Iine at 9.6 I_tm P(20) are calculated according to the spectroscopic constants given by Freund et al. [lo], and are summarized in table 1. Here, the frequencies
are much less dependent
on
the K value since only the transition of AK = 0 is a!lowed. !nitially, CH,F is excited to the v3 level with J = 12 or J = 0 by absorbing one photon of the laser. E rotational transitions from J = I2 or O-8 in the u = 1 level of the u3 mode occur by collisions with Ar, one more photon is absorbed to excite CH,F in the v3 level to 2~3. Further excitation is possible from this level to 3~3 through the qR(20) transition. Owing to the anharmonicity, the absorption line resonant with the laser line is found only from a high rotational level for the v = 3 + 4 transition, i.e. J = 35. There-
324
b
10
20
30
40
Time/ ~5 Fig. 2. Laser-induced fluorescence of the ~3 overtone ob served through the monochromator with an energy thence of 1.4 J cm--2 in a CH3F/Ar mixture of p(CH3F) = 0.06
and p(Ar) = 70 Torr. The monochromator with its spectral sliiIit width of 0.0385 pm is set at 4.76 (a), 4.84 (b) and 4.92 rrm (CL and the relative sensitivity af the detecting sy~em is 1 : 1 : 2 for (a), (b) and (c).
Volume 84; number 2
CHEMICAL
PFEYSICSLE’lTERs
1 December 198i
time/us
Fitz-3. Pomilation changes in the 2~~. 39 and 415 levels de* temrined from the data shown in fig. 2. intensity increases rapidly to a certain level that decays dowly. While, it is found from fig. 2b that the emission at 4.84 m, to which both the u = 2 + 0 and u = 3 -t 1 transitions contribute?, indicates a rapid rise of the emission which is followed by a fast and a slow decay. The relative population in the 3v3 level can be determined on the basis of the emission in fig. 2b, from which one must subtract the emission in fig. 2a which should be corrected by a factor representing the relative sensitivity of the emission from the 2v, level at 4.76 m to that at 4.84 p. Fig. 3 shows the result of such a deconvolution of the data in fig. 2. In the calculation, it is assumed that the vibrational transition probability of u + u - 2 is proportional to u2 _ Ihe population in the 4~3 level could not be determined from fig. lc except the very initial stage of the excitation, since the contribution of the u = 4 -+ 2 emission was found to be less than the noise level. In fig. 3, it is seen that the population in the 3~3 level increases to a peak with a very large rate just after the laser pulse and decays quickly to a certain level, while the population in the 2v, level increases with the same rate to a value which decays very slowly. The time history of the population in the 39 level is very similar to that of the v4 and nei~bo~g levels. This supports the conclusion derived earlier [S] that intermode V-V energy trans-
Fig. 4. Energy level diagram describing the in&s- and intermode V-V
transfer in CH3F.
The figure zzc&md
to ea&
process is the time constant in * for a mkture of 60 mTorr CH3 and 70 Ton AI; the rate constants employed to estimate the time constant are from ref. [61 (a), ref. [21 (b). those estimatedby the Landau--letter rule based on the data in ref. 161 fc) and in ref. 121 cd), and the present estimation (e). fer between the 3v, and v4 levels is very fast. In a lean mixture of CHSF in Ar of high pressure, the ladder chmbing intermolecular intramodc V-V transfer is much slower than the in~olecukr btermode V-V transfer. In fig. 4 is shown the schematic diagram of the energy flow of CH3 F below the 3~9 level. In fig. 4, a tune constant for each process is defined as If(k, + k_)[Ar] , where k+ and k_ sre the V-V rate constants for the forward and backward directions, respectively, and the one for the ladder climbing is l/kf,“u_ 1 [CHsF] where k~~~_z is the
v3 intramode
V-V
transfer rate constant.
Re-
ferring to fig. la, in pure CH, F of 60 mTorr, the initial excitation to the v3 Ievel is followed by +&eintrarnode V-V process to reach a qu~i~q~b~urn distribution within the v3 mode, and later, the intermode V-V transfer of v3 * v6 + v2,vs occurs with a time constant of =lOO ps, and fmally a quasi-equilibrium distribution among all modes is established before V-T/R deactivation starts. This picture of the energy flow should not be applied to tile present case. Firstly, the direct excita325
Volume 84. number 2
CHEhlICAL
tion up to the 3~3 level occurs through collisionassisted multiple photon absorption_ Secondly, as described in fig. 4, the intermode V-V transfer dominates the intramode ladder climbing V-V. Thus, the initial rapid decay of the 3v3 population is attributed to the intramolecular intermode V-V transfer from tJx 3~3 to the 2~3 level via many intermediate levels,
1 December
PHYSICS LE-l-I-ERS
1981
transfer described in fig. 4. This would be the reason for a second slow rise of the 2~~ emission not to be interpreted simply due to ladder climbing V-V energy transfer. A detailed analysis based on the coupled rate equations is necessary to evaluate quantitatively the role of third-body collisions in the intramolecular energy
flow.
and at a later stage. if the “up-the-ladder” intramode V-V
transfer
begins, an energy
transfer
cyclic path
starting from the 3~3 through the 2v3 level may be realized. This cyclic path is referred to as “catastrophic” [3], since one IQ vibrational quantum is released into the translational and/or rotational degrees of frcedonl during one cycle. The vibrational energy stored in CH,F in levels between the 3~ and 2~3
levels is transferred to translation and/or rotation Lhroufl~ this cyclic path with n rate much larger than the V-T/R rate. until the vibrational temperature defined by the relative populations in the levels between 3u, and 71’~ reaches the transiational/rotational temperature. The rapid decay of the 3v3 population found in fig. 3 would be due to this “catastrophic” cyclic path. Another path is possible between the 2v3 and u3 levels, though this path isstable since the vibrational quanta are kept constant. Therefort,
the 2u3 and u3 populations must be reduced
by the V-T/R tl~r result such
as the cast
transfer
326
shown
transfer.
and this is in accord
in fig. 3. In a misture of fig. 1 b, the ladder
is competitive
with
with
less Ar
climbing
with the intermode
V-V
V-V
References [l] G.W. Flynn. in: State-to-state chemistry. eds. P.R. Brooks-&d E.F. Hayes (Am. Chem. SOIL:, Washington, 1977)
p. 145.
121 V.A. Apkarian and E. Weitz, J. Chem. Phys. 71 (1979)
4349. I31 h1.L. Mandich and G.W. Flynn, J. Chem. Phys. 73
(1980) 3679. 141 VS. Letokhov
and C,B. Moore, in: Chemical and biochemical applications of lasers, ed. C.B. Moore (Academic Press, New York, 1977) p. 1.
151 H. Nakane, Dissertation, University of Tokyo (1981). IsI R.S. Sheorey and G.W. Flynn, J. Chem. Phys. 72 (1980)
1175.
[71 hl. Kneba and J. Wolfrum, Ber. Bunsenges. Physik. Chcm. 81 (1977)
1275.
ISI H. Nakanc and S. Tsuchiya,
Chem. Phys. Letters 80 (1981) 458. PI hl. Kneba, R. Stender, U. Wellhausen and J. Wolfrum, J. hLol. Struct. 59 (1980) 7-07. 1101S.hl. Freund, G. Dusbury, hl. Roheld, J.T. Tiedjo and T. Oka, J. hfol. Spectry. 52 (1974) 38. [111 T.Y. Chang, Optics Commun. 2 (1970) 77.
84.
number
COLLISION-ASSISTED BY IRRADIATION Hide&i
NAKANE
1 December 1981
Cl-lEMiCAL PIIYSK!S LETTERS
2
MULTTPLE
STEP EXCITATION
WITH A TEA CO,
OF CH3F
LASER *
and Soji TSUCHlYA
Received 5 May 1981; in final form 17 August 1981
By irruditlting a mixture of CH3F highly diluted In Ar with ;I TEA CO2 laser, the ~3 overtone emission rises with a rate much larger than the “up-the-ladder” V-V pumping rate indicating that the CH3F is exited up to the 3~3 level almost instantly. This occurs via multiple step excitation assisted by rotational transitions in collisions.
1. Introduction According to the general scope on the mechanism of the vibrational energy flow in molecular species [ 11, if the species is diluted in 3 monatomic gas such as Ar, the role of coflisions is to enhance intramolecuinr intermade vibration-to-vibration (V-V) energy transfer. Thus, when laser photons are absorbed in one vibrational mode, V-V energy transfer from the excited level of the pumped mode to other nearresonant levels occurs to establish a quasiequilibrium distribution among these levels. This type of energy flow mechanism induced by third-body colhsions is confirmed in laser fluorescence studies, for example, on the CH3F/Ar mixture 123 and on the OCS/inertgas mixtures [3 ] . In this letter, we introduce a different type of collision effect: a collision-assisted multiple step excitation; the 2~3mode of CH3 F in Ar is excited to higkrer levels by jrradiation with a strong CO, laser. This intramolecLlar intramode excitation differs from the “up-the-ladder” intramode V-V energy trarrsfer [1] or the IR multiple photon excitation induced by strong laser irradiation [4].
2. Experimental A TEA CO, Iaser (Lumonics 102-2) beam whose section was reduced to 4 by a condenser lens was introduced into a fluorescence cell. The pulse shape of the laser was composed of a spike of 200 ns fwhm and a tail of I .S MSduration when the laser medium was a He/COZ/NZ (3 : 1 : 0.4) mixture, and the maximum fhence of the Iaser was 1.5 .I cme2 (9.6 ym, P(20)]. The cell contained two parallel mir-
cross
rors 12 cm apart from each other to reflect
the laser
of the beam being ==48 cm. The laser-induced fhrorescence collected by a Wet&type mirror system was detected by an InSb detector grout an appropriate IR fitter or a monochromator. The response time of the detector including a preamplifier was ~2 ps. The signal formed by each laser pulse was digitized, and 1024 or 2048 signals were averaged for noise reduction. The sample 8as of CH,F (PCR, research grade) was employed after repeated solidification, and Ar (9Q_999~ pure) was treated by passing through two successive columns of copper chips at 200°C and of molecular sieve SA cooled to -114S”C. Agiven fraction of CH3F iu Ar was prepared by mixing the respective gases of known pressures in vessels with a calibrated volume ratio.
beam
three times. the path length
* This work has been supported in part by a Grant-in-Aid from the Ministry of Education (No. 311606).
322
0 009-2614/81/0000--0000/S
02.75 0 1981 North-Holland
Volume 84, number 2
CHEMICAL PHYSICS LE-ffERS
3. Results and discussion Figs. la-Id show the observed time histories of the overtone emission of CH~ F irradiated with a TEA CO, laser pulse. In fig. I, the emission data obtained in pure CH, F and CH3 F/Ar mixtures with a CH3 F partial pressure of 60 mTorr are compared with each other. The emission in pure CH,F rises slowly to a rather weak broad peak, while when Ar is added to a pressure around 10 Torr_ the rise of the
emjssion becomes too fast to be followed by the present detector system and a peak appears. Following this peak the emission shows a second slow rise to form a broad peak. This difference between the data in figs. la and 1b couId not be attributed to ;I lower laser energy fuence for irradiating pure CHsF than for the mixture, since the rise of the v3 overtone emission in pure CH, F at a pressure lower than 0.1 Torr is much less dependent on the fluence than in a mixture with Ar [S] . In a mixture co~t~~~ more Ar, the initial peak becomes clearer though the second broad peak is hard to recognize. if the initial rise of the emission
Fig. 1. ~uoresce~~ observed dour a wide-band Biter of the 5 pm region in pun? C&F (a) and CHs/A.r mixtures (b. c, d) and that of the 3 pm region (e) induced by the laser icradiation with an energy ffuegce of 0.4 J cmB2 (a).and 1.2 J cm-’ (b, c, d, e). The partial,pressure of CHSF is 0.06 Torr and those of AI are 0 (a), 7.7 @), 19.3 (c) and 70 Torr (a,@. The relative sensitivity of the detecting system for (a)-(e) &16:2:1:1:0.63.
I Decemberi98L
is defmed approximately by a functional form of 1 -e-1/r, the time constant 7 observed in pure CH,F is close to the value of 6.7 ps which is given by -&e “up-the-ladder” rate constant determined by Sheorey and FIRM [6], while the time cons~ts found in the CH3 F/Ar mixtures are much tess than 6.7 ps. Thus, it is concluded that the excitation of the ~3 mode in a CH,F/Ar mixture must be caused by a mechankm
different
from “up-the-ladder”
V-V
energy transfer and that the excitation is assisted by collisions with Ar. Besides this new excitation mechanism, the ~~p-~e-Iadder’~ V-V
pumping of the p3 mode occurs simultaneously resulting in a slow second rise of the emission. However, this rise is found only in mixtures at a pressure lower than -20 Torr, and as the pressure of Ar increases, the second peak appears closer to the time origin. Since the coexistence of Ar may not affect the ladder climbing rate, some other mechanism in addition to the intramode “up-the-ladder”
V-V
transfer nxxst be taken into
account for a full understanding of the intra- and inter-mode energy flow in a mixture of CH3 F higbly diluted in Ar. In order to test the possibility of direct excitation to the 2~3 level by two-photon absorption of the CO, laser, which was formerly suggested by Kneba and Wolfrum 171, the p3 mode of CH3 F was excited by two types of laser pulses; one was composed of a spike with a tail and the other was only a spike, which was vb&ajned from the laser medium without Nz. When the laser energy ffuences of two pulses were of the same magnitude, the peak power of the pulse with the taii was nearly half the pulse without the tail. Nevertheless, the observed emissions induced in the CH3FjA.r mixture at 70 Torr by both laser puises were the same, indicating that the excitation of the v3 mode of CH,F in Ar is not due to a multiple photon absorption. The observed emissions of the 5 and 3 m regions in the CH,F/Ar mixture of 70 Torr are shown in figs. Id and ie, respectively. The 3 trm emission rises rapidly with a rate limited
by the detector
response,
and reaches a pee which exhibits a fast decay foilowed by a slow one. The~observed rapid rise of the emissions in both the 3 and 5 w regions may evidence a very fast intermode V-V energy transfer between the 3~~ level and, the v4 + neighbqring levels as qlready pointed out by the authors in a previ323
Volume 84. number 2
C~ICAL
PHYSICS
LE-ITEFtS
1 December 1981
Table I Frequencies of paragon-ro~tion
a) The
transitions of the ~3 mode near-resonant with the CO2 Iaser iine at 9.6 Brn P(20)
Vi”viationa1 transition
Rotational transition
Wavenumber (R = 0)
1-O
qQK (11)
1047.122
8.04
qQK (121
1046.853
0.03
~QK (131
1046.560
8.81
qPx (1)
1046.907
1.59
2-l
qR~ (8)
1046.91
1.71
3-z
QRK (20)
I046.975
3.63
1046.618
7.08
4-3 qRK (35) -__-- ---. ..- -_~. wavenumber of the laser iine is 1046.854 cm-’
[ 11)
18). Kneba et al. 191 also found a rapid rise of the emission from the iq , ~4 levels induced by a TEA CO2 laser, and concluded efficient V-V energy transfer between the 3u3 and these levels. However, they speculaIed that the increase of the rise rate of the 3 pm emission by addition of Ar was due to enhancement of intermode V-V energy transfer which leads to the energy flow u3 + v6 -+ “2, vs Y4 besides v3 + 2~3 -+ 3u3 4 vl, “4. -+2Y2,205+v~, Since the Ar pressure in their experiment was insufexcitation
)
I
Non-resonance ldvl (GHz) a)
_
011spaper
ficient, the collision-assisted
(cm-’
mechanism
here may not dominate the intramode or ~te~odc V-V energy transfer by self-collisions of CH, F. proposed
fore, the 45 level wotifd be hard to attain due to the very small
population
in such a high rot~+tiond transition rate is on the order of 107--lo6 s-l Torr-I, an Ar pressure > 10 Torr is sufficient for the rotational transitions within the laser pulse duration to cause multiple step excitation. The time-dependent poptdation distribution in each level of the v3 mode may be discussed on the basis of the v3 overtone emission spectrum observed level. Since the rotational
of a monochromator. Fig. 2a shows the emis4.76 #rn which is interpreted as due to the u = 2 * 0 transition of the v3 overtone. The emission by use
sion at
The frequencies of the ~bration-rotation transitions of *he v3 mode which are near-resonant with the CO, laser Iine at 9.6 I_tm P(20) are calculated according to the spectroscopic constants given by Freund et al. [lo], and are summarized in table 1. Here, the frequencies
are much less dependent
on
the K value since only the transition of AK = 0 is a!lowed. !nitially, CH,F is excited to the v3 level with J = 12 or J = 0 by absorbing one photon of the laser. E rotational transitions from J = I2 or O-8 in the u = 1 level of the u3 mode occur by collisions with Ar, one more photon is absorbed to excite CH,F in the v3 level to 2~3. Further excitation is possible from this level to 3~3 through the qR(20) transition. Owing to the anharmonicity, the absorption line resonant with the laser line is found only from a high rotational level for the v = 3 + 4 transition, i.e. J = 35. There-
324
b
10
20
30
40
Time/ ~5 Fig. 2. Laser-induced fluorescence of the ~3 overtone ob served through the monochromator with an energy thence of 1.4 J cm--2 in a CH3F/Ar mixture of p(CH3F) = 0.06
and p(Ar) = 70 Torr. The monochromator with its spectral sliiIit width of 0.0385 pm is set at 4.76 (a), 4.84 (b) and 4.92 rrm (CL and the relative sensitivity af the detecting sy~em is 1 : 1 : 2 for (a), (b) and (c).
Volume 84; number 2
CHEMICAL
PFEYSICSLE’lTERs
1 December 198i
time/us
Fitz-3. Pomilation changes in the 2~~. 39 and 415 levels de* temrined from the data shown in fig. 2. intensity increases rapidly to a certain level that decays dowly. While, it is found from fig. 2b that the emission at 4.84 m, to which both the u = 2 + 0 and u = 3 -t 1 transitions contribute?, indicates a rapid rise of the emission which is followed by a fast and a slow decay. The relative population in the 3v3 level can be determined on the basis of the emission in fig. 2b, from which one must subtract the emission in fig. 2a which should be corrected by a factor representing the relative sensitivity of the emission from the 2v, level at 4.76 m to that at 4.84 p. Fig. 3 shows the result of such a deconvolution of the data in fig. 2. In the calculation, it is assumed that the vibrational transition probability of u + u - 2 is proportional to u2 _ Ihe population in the 4~3 level could not be determined from fig. lc except the very initial stage of the excitation, since the contribution of the u = 4 -+ 2 emission was found to be less than the noise level. In fig. 3, it is seen that the population in the 3~3 level increases to a peak with a very large rate just after the laser pulse and decays quickly to a certain level, while the population in the 2v, level increases with the same rate to a value which decays very slowly. The time history of the population in the 39 level is very similar to that of the v4 and nei~bo~g levels. This supports the conclusion derived earlier [S] that intermode V-V energy trans-
Fig. 4. Energy level diagram describing the in&s- and intermode V-V
transfer in CH3F.
The figure zzc&md
to ea&
process is the time constant in * for a mkture of 60 mTorr CH3 and 70 Ton AI; the rate constants employed to estimate the time constant are from ref. [61 (a), ref. [21 (b). those estimatedby the Landau--letter rule based on the data in ref. 161 fc) and in ref. 121 cd), and the present estimation (e). fer between the 3v, and v4 levels is very fast. In a lean mixture of CHSF in Ar of high pressure, the ladder chmbing intermolecular intramodc V-V transfer is much slower than the in~olecukr btermode V-V transfer. In fig. 4 is shown the schematic diagram of the energy flow of CH3 F below the 3~9 level. In fig. 4, a tune constant for each process is defined as If(k, + k_)[Ar] , where k+ and k_ sre the V-V rate constants for the forward and backward directions, respectively, and the one for the ladder climbing is l/kf,“u_ 1 [CHsF] where k~~~_z is the
v3 intramode
V-V
transfer rate constant.
Re-
ferring to fig. la, in pure CH, F of 60 mTorr, the initial excitation to the v3 Ievel is followed by +&eintrarnode V-V process to reach a qu~i~q~b~urn distribution within the v3 mode, and later, the intermode V-V transfer of v3 * v6 + v2,vs occurs with a time constant of =lOO ps, and fmally a quasi-equilibrium distribution among all modes is established before V-T/R deactivation starts. This picture of the energy flow should not be applied to tile present case. Firstly, the direct excita325
Volume 84. number 2
CHEhlICAL
tion up to the 3~3 level occurs through collisionassisted multiple photon absorption_ Secondly, as described in fig. 4, the intermode V-V transfer dominates the intramode ladder climbing V-V. Thus, the initial rapid decay of the 3v3 population is attributed to the intramolecular intermode V-V transfer from tJx 3~3 to the 2~3 level via many intermediate levels,
1 December
PHYSICS LE-l-I-ERS
1981
transfer described in fig. 4. This would be the reason for a second slow rise of the 2~~ emission not to be interpreted simply due to ladder climbing V-V energy transfer. A detailed analysis based on the coupled rate equations is necessary to evaluate quantitatively the role of third-body collisions in the intramolecular energy
flow.
and at a later stage. if the “up-the-ladder” intramode V-V
transfer
begins, an energy
transfer
cyclic path
starting from the 3~3 through the 2v3 level may be realized. This cyclic path is referred to as “catastrophic” [3], since one IQ vibrational quantum is released into the translational and/or rotational degrees of frcedonl during one cycle. The vibrational energy stored in CH,F in levels between the 3~ and 2~3
levels is transferred to translation and/or rotation Lhroufl~ this cyclic path with n rate much larger than the V-T/R rate. until the vibrational temperature defined by the relative populations in the levels between 3u, and 71’~ reaches the transiational/rotational temperature. The rapid decay of the 3v3 population found in fig. 3 would be due to this “catastrophic” cyclic path. Another path is possible between the 2v3 and u3 levels, though this path isstable since the vibrational quanta are kept constant. Therefort,
the 2u3 and u3 populations must be reduced
by the V-T/R tl~r result such
as the cast
transfer
326
shown
transfer.
and this is in accord
in fig. 3. In a misture of fig. 1 b, the ladder
is competitive
with
with
less Ar
climbing
with the intermode
V-V
V-V
References [l] G.W. Flynn. in: State-to-state chemistry. eds. P.R. Brooks-&d E.F. Hayes (Am. Chem. SOIL:, Washington, 1977)
p. 145.
121 V.A. Apkarian and E. Weitz, J. Chem. Phys. 71 (1979)
4349. I31 h1.L. Mandich and G.W. Flynn, J. Chem. Phys. 73
(1980) 3679. 141 VS. Letokhov
and C,B. Moore, in: Chemical and biochemical applications of lasers, ed. C.B. Moore (Academic Press, New York, 1977) p. 1.
151 H. Nakane, Dissertation, University of Tokyo (1981). IsI R.S. Sheorey and G.W. Flynn, J. Chem. Phys. 72 (1980)
1175.
[71 hl. Kneba and J. Wolfrum, Ber. Bunsenges. Physik. Chcm. 81 (1977)
1275.
ISI H. Nakanc and S. Tsuchiya,
Chem. Phys. Letters 80 (1981) 458. PI hl. Kneba, R. Stender, U. Wellhausen and J. Wolfrum, J. hLol. Struct. 59 (1980) 7-07. 1101S.hl. Freund, G. Dusbury, hl. Roheld, J.T. Tiedjo and T. Oka, J. hfol. Spectry. 52 (1974) 38. [111 T.Y. Chang, Optics Commun. 2 (1970) 77.