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Laser induced fluorescence of NCS in the gas phase
Volume 111, number 1,2
26 October
CHEMICAL PHYSICS LETTERS
1984
LASER INDUCED FLUORESCENCE OF NCS IN THE GAS PHASE Hirokazu Kazuhiko
OHTOSHI, SHIBUYA,
Koichi
TSUKIYAMA,
Akihiko
YANAGIBORI,
Kinichi OBI and Kuzo TANAKA
Departtnent of CI~emistry. Tokyo Insritute of Tecfmology, Ofzokayanuz.Meguro. Tokyo 1.52. Japan
Received 20 July 1984
The near-ultraviolet bands of NCS have been studied in the gas phase by laser induced fluorescence. The fluorescence spectrum showed Fermi interaction between the 001 and 020 levels in the ground X ‘fIi state. The fluorescence lifetime of the 000 level of the excited A zni state was 164 f 10 ns. The collisional relaxation between the spin-orbit sublevels in the A ‘ni state was found to take place at almost the same rate as the gas kinetic rate. The energy separation between the Fr and F2 sublevels of the A ‘flf state was dctermincd to be 138 + 10 cm-‘.
1. Introduction The NCS free radical was first observed in emission by Holland et al. [I] in the vacuum ultraviolet photolysis of isothiocyanate and thiocyanate compounds. Dixon and Ramsay [2] analyzed the absorption bands of NCS between 330 and 400 nm and assigned them to two electronic band systems. A “11,-X ‘Hi, with its origin at 26054 cm-l, and B 2C+-X ?fli, with its origin at 26844 cm-l. They also found that there was con-
siderable evidence for vibronic interaction between the A 211j and B ‘C+ states. Recently, the dynamic behavior of the corresponding free radical, NCO, has been extensively studied in an Ar matrix by Bondybey and English [3], and in the gas phase by Charlton et al. [4] and Sullivan ct al. [S] . In comparison with NCO, however, there is little kinetic information on NCS. in this paper, we report the study of laser induced fluorescence of NCS radicals generated by the 193 nm photolysis of CH, NCS.
The dyes used were BBQ and PBD (both from Exciton Co.) which covered the excitation wavelengths of 363390 nm. The two laser beams were brought into a stainless steel cell through quartz windows on opposite sides and were aligned so as to overlap spatially. The delay time between the two lasers was electronically controlled by a pulse generator_ The repetition rate was 10 pulse/s throughout the experiment_ The fluorescence of NCS was observed at rigtrt angles to the excitation laser beams. A monochromator (Nikon P-250)/photomultiplier (Hamamatsu lP28) combination was used to take the fluorescence spectra. A digital memory (Iwatsu DM901, IO ns resolution) interfaced to a microcomputer (Sharp MZ-SOB) was used to measure the fluorescence lifetimes. CH,NCS (Kant0 Chem. Co.) was degassed at 77 K. Research
3. Results
2. Experimental NCS radicals were generated by the photolysis of CH,NCS by an 193 nm ArF excimer laser (Lambda Physik EMG-103E). The probe laser was a nitrogen laser (Molectron UV-3-4) pumped dye laser (DL-I4P). 136
grade N2 (Takachiho
Co.) was used without
further purification.
and discussion
The fluorescence excitation spectrum for the A ‘nj -X 2nj and B ‘?Y--X ‘“j systems of NCS is SIIOWII in fig. 1, where the delay time between the two laser pulses was 5.5 US and CH,NCS (0.7 mTorr) was diluted 1O4 times with N2 (7 Torr). The vibrational assignments was carried
out based
on those
from the absorption
Kg. I. Fluorcseencc excitation spectrum of NCS at 363-387 nm. The deky time between if 1% nm photolysis Iascr pulse and a visible probe laser pulse was $5 gs, and CHJNCS (0.7 mTorr) Wasdiluted lo4 times with N2 (7 Torr). spectrum as determined by Bxon and Ramsay p]_ The observed fh~orescence excitation spectrum agrees weif in intensity with the absorption spectn.m reported by them, which indicates that the fluorescence quauturn yield is constant over this spectral range and that
380
400
420 Wavelength
predkodation
probably does not ocCur in this energy gas, the excitation spectrum taken at a short delay time after the 193 nm photolysis was complicated due to the congestion of both vibrationally and rotationally hot bands. The exci-
region_ In the absence ofdiluent
440
460
480
( nm )
Fig. 2. FIuoresCenCC spectrumof NCS A 2n#IOO) to X 2ni. me delay time bCt!Vecn the photolysis and probe laser pulses was 50 ps, and NCS was ex=citadat the band head of the A 2~,~2
Volume 111, number 1,2
tation spectrum became simpler with a longer delay time and the higher pressures, due to vibrational and rotational cooling in the ground state of NCS as a photoproduct._The delay time of 50 ps was almost long enough to eliminate the hot bands in the observed excitation spectrum even at a total pressure of 100 mTorr. Under our experimental conditions, the NCS radical seems to be rather stable chemically
26 October 1984
CHEMICAL PHYSICS LEITERS
because NCS was
Table 1 Locations and assignment of NCS A ‘~i(OOO) fluorescence bands Transition
Location
Lower level a)
Our0
(cm-’ 1
382.2
WU,
Gv,l)
detectable even at a long delay time such as 10 ms under similar pressure conditions_ On the 10 ms time
385.6 392.4
scale, the NCS radicals diffuse out of the probe dye
393.4
26164 25934 25484 25419
396.4
25227
403.7 404.6 406.0 408.2 409.2 412.7 416.7 425.2 4265 429.8 430.8 438.1 439.6 440.9 443.2 444.7 448.2 453.0 463.0 464.8 468.3 469.7
24771 24716 24631 24498 24438 3423 1
@*0.3
23998
U.O,O)
23518 23447 23267 23213 22826 22748 22681
~l,O,l)
laser beam and the LIF signal disappears. Fig. 2 shows the emission spectrum of NCS observed by exciting the radicals at the band head of the A 2Il3,2(000) + X*ff3,2(000) transition. The emission spectrum consists of the FI(A 2fI312 + X 2T1312) and F,(A 2f11,, + X ‘fIt,z) transitions_ This indicates that reiaxation between the A 2fI312 and A 2fll,, sublevels occurs during the lifetime of the excited states. Therefore, the intensity ratio of the FI to F2 transitions is dependent on the total pressure employed. This coiIisional relaxation between spin-orbit sublevels will be discussed later more in detail. The CN stretching (v’;) progressions are assigned as shown in fig. 2 and the frequency of v; vibration has been determined for the first time to be 1930 cm-l. The bending (~5) and the CS stretching (VI;) progressions cannot be straight. forwardly assigned from the spacings and the intensity pattern. Since the VI; frequency is almost twice the I$ frequency, Fermi interaction is expected to occur between two vibrational levels of 001 and 020. Fermi resonance is also expected to occur among the three levels, 002,07 I and 040. The transition energies and their assignments, including the Fermi resonances, are listed in table 1_ The fluorescence lifetimes for A 211i(OOO) were measured at total pressures between 0.1 and 10 Torr. No difference was found between the lifetimes of 2*312 and 2IiIt2_ The lifetimes were found to be independent of pressure over the range employed, either.
The measured fluorescence lifetime was 164 f 10 ns, which gives an oscillator strength of f -3 X low2 for the A 21$-X 2ffi transition of NCS assuming that the nonradiative process is negligible. The lifetime of NCS A 2 Hi is longer than that of NC0 B 2fI of 63 ns [s]. As described above, relaxation takes place between the spin-orbit sublevels in the upper state during the lifetime of the excited states. A simple two level model 138
KLo,w KMJ.1) iO,Z,O~ @,ZO)
KL2.1)
21598 2151s 21353 31290
3 3
(0.4.0) 3 (I .O,O)
~1.2.0~3 uxk1t (12.0)
1
(l&2) (12.1)
(1,4.0) 1
22563 22487 22311 22075
I
3 f2,W) Gw,0) cw.1) (2.2.0)
3 (uh1) (~.2,0)3
a! Braces: perturbed by Fermi interaction.
is considered as shown in fig. 3a. Since the fluorescence lifetime of NCS is independent of the total pressure, collisional electronic quenching can be neglected. Thus, the time evolution of the NCS concentrations in the A 2II3,2 and A 2fI1,2 state can be formulated as follows,
EFJ, = {iFI lol(k12+ k2,)3O$ wC--kfQ
Volu&
26 October
CHEMICAL PHYSKS LE7TERS
111, number 1,Z
a)
1984
f Fzl A*Th2
The ratio The slope the slope tained by
kt kt
-i
-
XZTrlfZ
oflFI/IrZ2 is plotted against [M]-l in fig. 3b. gives kf/ki2 and the intercept ka,/k,~. From and intercept, the following values are oba least-squares method:
fit&2
= (4.40 2 030)
!%#p _
_ = 1.95 f 0.07.
X f Ox6 molecule
cm-‘,
Using the value of kf, (6.11 + 0.36) X I O6 s-* , the redistribution rate constants are determined to be
W
k 13 = (139
+ 025)X
10-l’
cm3 n~olecule-1
s-l,
cm3 molecUle-1
s-l _
and kzr = (Z-71 * 0.383 X lOdo
&
(
cm3 motet-’
1
Fig. 3. (a) A reiaxa?ion model between spin sublevels.kfrepresents a radiative rate constant.klz and R21 are F1 -. FZ and FZ -+ IF, redistribution rate constants, respectively. [hi J is the concentration of the collision partner which is essentially equal to the N2 pressure in the present case. [F1] and [Fz] are the NCS concentrations in A ‘r1,1~(000) and A 2nIlz(OOO), respectively. @) Plots of ZFI/ZFz versus [~&I]-‘.
and
-
=P
[-($2
[W
+ k,,
WI
+
k$l3,
where [F~]O is the initial F, concentration at t = 0. Since the boxcar gate was opened for about I_.Ls and the fluorescence lifetime was about. 165 ns, the intensity of F, fhOreSC&Ice (IF,) is proportional to the timeintegrated population of [Fl ]r over t = 0 to m. In this case, then ratioIF,/IFe can be expressed as follows:
Thus, the collision-induced redistribution between the spin-orbit sublevels in the upper state @cl2 + h-11= 4.1 X lo-10 cm3 molecule-l s-l) takes place very fast, at almost the same as the gas kinetic rate. If collisional redistribution leads to thermal equilibrium between the spin-orbit sublevels, the ratio of k12/k21 should give the Bokzmann factor, exp(-AE,2/kT), where &El2 is the energy separation between the F, and F2 sublevels. As a result, one obtains AEIz = 138 -t IO cm-l, which agrees with LLE12 = 125 + 20 cm-l derived from the spin-uncoupling procedures of the O-O absorption band of NCS A zIIj-X ‘Ili [2] _To the best of our knowledge, little information is available on the collisional relaxation between spin-orbit sublevels. Sudbo and Lay [6] measured the rate constants for collision-induced spin-orbit and rotational transitions in NO X 2 Ilr (u = 2). The transition rates betwecn the spin-orbit sublevels, separated by 120 cm-l, are found to be comparable to the rotational relaxation rates and the sum of both rate constants is 39 X lo-l1 cm3 molecu@ s-l for NO-N* collisions. Thus, the collision-induced spin-orbit transitions in NO X 2 II, occur about 10 times less efficiently than those in NCS A”& although the values of AE12 are almost the same.
139
Volume 111. number I,2
CHEMICAL PHYSICS LmERS
References [l] R. Holland, D.W.G. Style, R.N. Dixon and D.A. Ramsay, Nature 182 (1958) 336. [I] R.N. Dixon and D.A. Ramsay, Can. J. Phys. 46 (1968) 2619. [3] VX. Bondybey and J.H. EngIish, J. Chem. Phys. 67 (1977) 2868.
140
26 October
1984
[4 ] T-R. Charlton, T. Okanura and B.A. Trush, Chem. Phys. Letters 89 (1982) 98. [S] B.J. Sullivan. GP. Smith and D.R. Crosley, Chem. Phys. Letters 96 (1983) 307. [6] AaS. Sudbo and h1.M.T. Loy, J. Chem. Phys. 76 (1982) 3646.
26 October
CHEMICAL PHYSICS LETTERS
1984
LASER INDUCED FLUORESCENCE OF NCS IN THE GAS PHASE Hirokazu Kazuhiko
OHTOSHI, SHIBUYA,
Koichi
TSUKIYAMA,
Akihiko
YANAGIBORI,
Kinichi OBI and Kuzo TANAKA
Departtnent of CI~emistry. Tokyo Insritute of Tecfmology, Ofzokayanuz.Meguro. Tokyo 1.52. Japan
Received 20 July 1984
The near-ultraviolet bands of NCS have been studied in the gas phase by laser induced fluorescence. The fluorescence spectrum showed Fermi interaction between the 001 and 020 levels in the ground X ‘fIi state. The fluorescence lifetime of the 000 level of the excited A zni state was 164 f 10 ns. The collisional relaxation between the spin-orbit sublevels in the A ‘ni state was found to take place at almost the same rate as the gas kinetic rate. The energy separation between the Fr and F2 sublevels of the A ‘flf state was dctermincd to be 138 + 10 cm-‘.
1. Introduction The NCS free radical was first observed in emission by Holland et al. [I] in the vacuum ultraviolet photolysis of isothiocyanate and thiocyanate compounds. Dixon and Ramsay [2] analyzed the absorption bands of NCS between 330 and 400 nm and assigned them to two electronic band systems. A “11,-X ‘Hi, with its origin at 26054 cm-l, and B 2C+-X ?fli, with its origin at 26844 cm-l. They also found that there was con-
siderable evidence for vibronic interaction between the A 211j and B ‘C+ states. Recently, the dynamic behavior of the corresponding free radical, NCO, has been extensively studied in an Ar matrix by Bondybey and English [3], and in the gas phase by Charlton et al. [4] and Sullivan ct al. [S] . In comparison with NCO, however, there is little kinetic information on NCS. in this paper, we report the study of laser induced fluorescence of NCS radicals generated by the 193 nm photolysis of CH, NCS.
The dyes used were BBQ and PBD (both from Exciton Co.) which covered the excitation wavelengths of 363390 nm. The two laser beams were brought into a stainless steel cell through quartz windows on opposite sides and were aligned so as to overlap spatially. The delay time between the two lasers was electronically controlled by a pulse generator_ The repetition rate was 10 pulse/s throughout the experiment_ The fluorescence of NCS was observed at rigtrt angles to the excitation laser beams. A monochromator (Nikon P-250)/photomultiplier (Hamamatsu lP28) combination was used to take the fluorescence spectra. A digital memory (Iwatsu DM901, IO ns resolution) interfaced to a microcomputer (Sharp MZ-SOB) was used to measure the fluorescence lifetimes. CH,NCS (Kant0 Chem. Co.) was degassed at 77 K. Research
3. Results
2. Experimental NCS radicals were generated by the photolysis of CH,NCS by an 193 nm ArF excimer laser (Lambda Physik EMG-103E). The probe laser was a nitrogen laser (Molectron UV-3-4) pumped dye laser (DL-I4P). 136
grade N2 (Takachiho
Co.) was used without
further purification.
and discussion
The fluorescence excitation spectrum for the A ‘nj -X 2nj and B ‘?Y--X ‘“j systems of NCS is SIIOWII in fig. 1, where the delay time between the two laser pulses was 5.5 US and CH,NCS (0.7 mTorr) was diluted 1O4 times with N2 (7 Torr). The vibrational assignments was carried
out based
on those
from the absorption
Kg. I. Fluorcseencc excitation spectrum of NCS at 363-387 nm. The deky time between if 1% nm photolysis Iascr pulse and a visible probe laser pulse was $5 gs, and CHJNCS (0.7 mTorr) Wasdiluted lo4 times with N2 (7 Torr). spectrum as determined by Bxon and Ramsay p]_ The observed fh~orescence excitation spectrum agrees weif in intensity with the absorption spectn.m reported by them, which indicates that the fluorescence quauturn yield is constant over this spectral range and that
380
400
420 Wavelength
predkodation
probably does not ocCur in this energy gas, the excitation spectrum taken at a short delay time after the 193 nm photolysis was complicated due to the congestion of both vibrationally and rotationally hot bands. The exci-
region_ In the absence ofdiluent
440
460
480
( nm )
Fig. 2. FIuoresCenCC spectrumof NCS A 2n#IOO) to X 2ni. me delay time bCt!Vecn the photolysis and probe laser pulses was 50 ps, and NCS was ex=citadat the band head of the A 2~,~2
Volume 111, number 1,2
tation spectrum became simpler with a longer delay time and the higher pressures, due to vibrational and rotational cooling in the ground state of NCS as a photoproduct._The delay time of 50 ps was almost long enough to eliminate the hot bands in the observed excitation spectrum even at a total pressure of 100 mTorr. Under our experimental conditions, the NCS radical seems to be rather stable chemically
26 October 1984
CHEMICAL PHYSICS LEITERS
because NCS was
Table 1 Locations and assignment of NCS A ‘~i(OOO) fluorescence bands Transition
Location
Lower level a)
Our0
(cm-’ 1
382.2
WU,
Gv,l)
detectable even at a long delay time such as 10 ms under similar pressure conditions_ On the 10 ms time
385.6 392.4
scale, the NCS radicals diffuse out of the probe dye
393.4
26164 25934 25484 25419
396.4
25227
403.7 404.6 406.0 408.2 409.2 412.7 416.7 425.2 4265 429.8 430.8 438.1 439.6 440.9 443.2 444.7 448.2 453.0 463.0 464.8 468.3 469.7
24771 24716 24631 24498 24438 3423 1
@*0.3
23998
U.O,O)
23518 23447 23267 23213 22826 22748 22681
~l,O,l)
laser beam and the LIF signal disappears. Fig. 2 shows the emission spectrum of NCS observed by exciting the radicals at the band head of the A 2Il3,2(000) + X*ff3,2(000) transition. The emission spectrum consists of the FI(A 2fI312 + X 2T1312) and F,(A 2f11,, + X ‘fIt,z) transitions_ This indicates that reiaxation between the A 2fI312 and A 2fll,, sublevels occurs during the lifetime of the excited states. Therefore, the intensity ratio of the FI to F2 transitions is dependent on the total pressure employed. This coiIisional relaxation between spin-orbit sublevels will be discussed later more in detail. The CN stretching (v’;) progressions are assigned as shown in fig. 2 and the frequency of v; vibration has been determined for the first time to be 1930 cm-l. The bending (~5) and the CS stretching (VI;) progressions cannot be straight. forwardly assigned from the spacings and the intensity pattern. Since the VI; frequency is almost twice the I$ frequency, Fermi interaction is expected to occur between two vibrational levels of 001 and 020. Fermi resonance is also expected to occur among the three levels, 002,07 I and 040. The transition energies and their assignments, including the Fermi resonances, are listed in table 1_ The fluorescence lifetimes for A 211i(OOO) were measured at total pressures between 0.1 and 10 Torr. No difference was found between the lifetimes of 2*312 and 2IiIt2_ The lifetimes were found to be independent of pressure over the range employed, either.
The measured fluorescence lifetime was 164 f 10 ns, which gives an oscillator strength of f -3 X low2 for the A 21$-X 2ffi transition of NCS assuming that the nonradiative process is negligible. The lifetime of NCS A 2 Hi is longer than that of NC0 B 2fI of 63 ns [s]. As described above, relaxation takes place between the spin-orbit sublevels in the upper state during the lifetime of the excited states. A simple two level model 138
KLo,w KMJ.1) iO,Z,O~ @,ZO)
KL2.1)
21598 2151s 21353 31290
3 3
(0.4.0) 3 (I .O,O)
~1.2.0~3 uxk1t (12.0)
1
(l&2) (12.1)
(1,4.0) 1
22563 22487 22311 22075
I
3 f2,W) Gw,0) cw.1) (2.2.0)
3 (uh1) (~.2,0)3
a! Braces: perturbed by Fermi interaction.
is considered as shown in fig. 3a. Since the fluorescence lifetime of NCS is independent of the total pressure, collisional electronic quenching can be neglected. Thus, the time evolution of the NCS concentrations in the A 2II3,2 and A 2fI1,2 state can be formulated as follows,
EFJ, = {iFI lol(k12+ k2,)3O$ wC--kfQ
Volu&
26 October
CHEMICAL PHYSKS LE7TERS
111, number 1,Z
a)
1984
f Fzl A*Th2
The ratio The slope the slope tained by
kt kt
-i
-
XZTrlfZ
oflFI/IrZ2 is plotted against [M]-l in fig. 3b. gives kf/ki2 and the intercept ka,/k,~. From and intercept, the following values are oba least-squares method:
fit&2
= (4.40 2 030)
!%#p _
_ = 1.95 f 0.07.
X f Ox6 molecule
cm-‘,
Using the value of kf, (6.11 + 0.36) X I O6 s-* , the redistribution rate constants are determined to be
W
k 13 = (139
+ 025)X
10-l’
cm3 n~olecule-1
s-l,
cm3 molecUle-1
s-l _
and kzr = (Z-71 * 0.383 X lOdo
&
(
cm3 motet-’
1
Fig. 3. (a) A reiaxa?ion model between spin sublevels.kfrepresents a radiative rate constant.klz and R21 are F1 -. FZ and FZ -+ IF, redistribution rate constants, respectively. [hi J is the concentration of the collision partner which is essentially equal to the N2 pressure in the present case. [F1] and [Fz] are the NCS concentrations in A ‘r1,1~(000) and A 2nIlz(OOO), respectively. @) Plots of ZFI/ZFz versus [~&I]-‘.
and
-
=P
[-($2
[W
+ k,,
WI
+
k$l3,
where [F~]O is the initial F, concentration at t = 0. Since the boxcar gate was opened for about I_.Ls and the fluorescence lifetime was about. 165 ns, the intensity of F, fhOreSC&Ice (IF,) is proportional to the timeintegrated population of [Fl ]r over t = 0 to m. In this case, then ratioIF,/IFe can be expressed as follows:
Thus, the collision-induced redistribution between the spin-orbit sublevels in the upper state @cl2 + h-11= 4.1 X lo-10 cm3 molecule-l s-l) takes place very fast, at almost the same as the gas kinetic rate. If collisional redistribution leads to thermal equilibrium between the spin-orbit sublevels, the ratio of k12/k21 should give the Bokzmann factor, exp(-AE,2/kT), where &El2 is the energy separation between the F, and F2 sublevels. As a result, one obtains AEIz = 138 -t IO cm-l, which agrees with LLE12 = 125 + 20 cm-l derived from the spin-uncoupling procedures of the O-O absorption band of NCS A zIIj-X ‘Ili [2] _To the best of our knowledge, little information is available on the collisional relaxation between spin-orbit sublevels. Sudbo and Lay [6] measured the rate constants for collision-induced spin-orbit and rotational transitions in NO X 2 Ilr (u = 2). The transition rates betwecn the spin-orbit sublevels, separated by 120 cm-l, are found to be comparable to the rotational relaxation rates and the sum of both rate constants is 39 X lo-l1 cm3 molecu@ s-l for NO-N* collisions. Thus, the collision-induced spin-orbit transitions in NO X 2 II, occur about 10 times less efficiently than those in NCS A”& although the values of AE12 are almost the same.
139
Volume 111. number I,2
CHEMICAL PHYSICS LmERS
References [l] R. Holland, D.W.G. Style, R.N. Dixon and D.A. Ramsay, Nature 182 (1958) 336. [I] R.N. Dixon and D.A. Ramsay, Can. J. Phys. 46 (1968) 2619. [3] VX. Bondybey and J.H. EngIish, J. Chem. Phys. 67 (1977) 2868.
140
26 October
1984
[4 ] T-R. Charlton, T. Okanura and B.A. Trush, Chem. Phys. Letters 89 (1982) 98. [S] B.J. Sullivan. GP. Smith and D.R. Crosley, Chem. Phys. Letters 96 (1983) 307. [6] AaS. Sudbo and h1.M.T. Loy, J. Chem. Phys. 76 (1982) 3646.