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Laser-induced fluorescence excitation spectrum of thiophosgene cooled by supersonic expansion
CHEMICAL
Volume 76, number 2
LASER-INDUCED COOLED
R. VASUDEV, Department
FLUORESCENCE
BY SUPERSONIC
Y. HIRATA,
of Chemistry.
Received 39 July 1980;
PHYSICS
EXClTATION
1 December
LETTERS
SPECTRUM
1980
OF THIOPHOSGENE
EXPANSION
EC. LIhl and WM. M&WIN
Wayne State University. Detroit, Michigan 182017. US.4
in final iorm 22 August 1980
The vIbrational structure in the laser-induced fluorescence excitation spectrum of the fust excited TT*.- n transition of thiophogene, seeded in a supersonic beam of argon, is reported. Hot bands are identified empirically by variation of beam temperature, and vibronic assignments are adjusted as required.
1. Introduction
2. Experimental
The visible x ‘A,-R ‘A, transition of thiophosgene, CIZCS, has been studied by many workers [l-3]. In the first comprehensive work, Brand et al. [I] established the vibrational analysis in considerable detail and identified the vibrational frequencies in the excited and ground states of 35CIZCS and 35C137ClCS_ FranckCondon analysis was used IO arrive at possible excited state structures. The high-resolution spectrum has resisted detailed rotational analysis due to strong overlap of bands of the two isotopic species and, in several regions, of hot and cold bands. In the only rotational analysis work to date, Lombardi [3] performed a contour analysis of a band near 5 142 A which was assigned by Brand et al. [l] to the 2;3;4; band of 35C1~CS. In this paper, we report the laser-induced fluorescence excitation spectrum of thiophosgene seeded in a supersonic beam of argon. The vibrational structure is fairly well resolved_ We have detected some hitherto unreported transitions and have empirically identified several hot bands by varying the vibrational temperature. These features have been reassigned as required. Laser-excited fluorescence in the isolated molecule has also been studied by Brus and McDonald 141, Brand et al. [S], and Lombardi et al. [6]. The results of Brus and McDonald imply state-independent fluorescence lifetime and quantum yield to the red of 4550 A (where photodissociation sets in). In one region of our excitation spectrum we notice, on the contrary, a statedependent quantum yield variation_
ClzCS (at its room temperature vapor pressure) was mixed with argon and expanded through a 100~ nozzle. the stagnation pressure of the argon being = 0.1-0.15 atm. The molecular jet was crossed with an unfocused pulsed laser beam from a Quanta-Ray model PDL-1 dye laser which was pumped by the third harmonic of a Nd : Yag laser operating at 10 Hz. The 03 cm-t fwhm, 5 ns, 75-500 kW peak power pulsed beam passed through a series of light baffles and intersected the molecuiar beam 5- 10 mm downstream from the nozzle. The fluorescence was filtered by sharp cut-off filters, detected by an RCA C7 15 IW photomultiplier, processed by a boxcar averaget and then recorded on ay-t recorder. A wavelength caIibration was established by recording strong features in the absorption spectrum of thiophosgene at room temperature with a very small portion of the Iaser beam. These were then compared with the published peak positions [ 1,2] _The measurements in these regions should be accurate to 0.5 cm-l. The positions of peaks in other regions of the escitation spectrum were then obtained by interpolation. Small portions of the excitation spectrum in the region 1868G-20350 cm-l were also recorded without the argon carrier gas. These scans involve higher vibrational temperatures and were invaluable in the identification of several spectra1 features as hot bands.
details
249
CHEMICAL
Volume 76. number2
PHYSICS
1 December 1980
LETTERS
Table 1 Excitation spectrum of thiophosgene a)
;;(cnl-‘)
iqcm-’ )
Assignment 35Cl+zS
Assignment 35Cl~CS
3%Y’CKS
ii(cm-’ ) %Y’cLcs
35c12cs
2:4;
19204.4 19206 9
3204;
19669.3
18690.9
4’:
19210.3
2;3;4:
19673.0
18706.0
4;6:
1a252.5
18688.5 18689.7
18708.0
19256.5
18714.9
46
19297.3
18715.8
3O43 I 0
19301.1
18729.7
19641.9 (c)
19674.1 31430 5 4,
19323.1
18731.8
3204:
19326.7
18785 (v.br)
3;4: b)
193909
Assignment
3;6:,
19675.7
2; 3:4;
19679.0
2847
19680.8
2;4:,
19684.1
2i3;4:,
197265 19727.7
2;3:4:,
19730.9
18849.2
19395.6
18894.8
19430.5
19733.2
2;3;41
18900.1
19433.0 (sh)
19734.6
2:3;4:
19434.2
19805.8 (c)
19435.0
19807.8 (c)
19437.4
19853.1
19004.7 (sh)
194409
19856.1
19006.0
19451.9
19868.6
18902.3
2; 3y4:,
18958.9 18961.1
3646
4:
19156.7
19871.1
32047
19476.2
19912.6
19481.1
19913.9
6;
19501.6
19916.0
19095.7 (c)
19502.7
19917.0 (h)
19097.8 (c)
19507.2
19919.9
19035.2 19038.9 19080.1 19081.9
19143.2 (sh) 191451
2$3?4!,
19148.8 19149.7 (sh) 19150.3
1;4y
19 162 (br)
19509.7
19922 (al)
19551.8
19927.0
19555.2
19930.2
19572.3
19983.1
19576.7
19985.0
3;4;6:,
1;3:4;
1;4; 310 203041
2;3;4:,
1;6;
19605.5
19190.0
19607.4
%3?4&2h4k
19192.6
19608.6
20098.7
19194.8
2;4:,
19620.5
19195.5
2;3:4;
19621.6
20100.6
1;2:4;
19636.4 (c)
2OlCj3.1
1;2;4:
19640.0 (c)
20111.4 (c)
19196.7 19203.6 250
2;3;4:
3V13’cK3s
Volume 76, number 2
CHEMICAL
PHYSICS
L December
LE-fTERS
1980
Table 1 (cant) I(cm-‘)
Assignment ssc12cs
20113.3 20116.9
1;3;4:,
2:3;4:,
20176.1
3scl~‘clCs 1;2;3;4:
20342.8
1204:
20345.2
1;2;3h4;
20317.7
1;2;3;4;
203040 1 4 I
20416.0
lb32041
i0516.1
1’04;b)
1:3x4;
20209.5
20579.7
20223.7
20582.3 (sh) 20590.4 1;2;3;4:,
20228.4 20229.7 20234.7
1;3;6:,
20765.2
20284.5 (c) 20287.0 (c) 20340.6
20822.7 20836.7 1
1541. z l;-:3&4?) -.
20998.4
21239.1
21280.6 212865
71313.0
1;2:4;
21320.9
1;2;3:4:,
21478.1
1;2;4:,
21472.2 21481.2
1$3A4:,
208 16.8
1;3;6h
3sc13’cIcs
21214.1
20409.8
20208.1
Wl2CS
21069.7
1;2;4;
20572.8
Assignment
21063.4 1;2;4;
20386.8
7’ 343’ -0 0 0
S(cm_’ )
3sc1zcs
1;3;4;
1;3;43,
20169 (sh)
Assignment ~____-
20381.8
2; 3;4:,
20164.6 20166.8
3scP’clCs
1;3;4:,
20153.2 20160.0
5(cm-*)
1;2;3;4:, 1:2;3;4; 1;2;3;4;
20994.2
21489.1 21536.8 21709.9
b) 152646
21717.9
a1 br = broad, sh = shoulder, c = cold band, h = hot band. b, The corresponding band of 35~37C~CS is hidden or below the noise level_
3. Results and discussion Table 1 presents the measured peak positions and their assignments *. Portions of the excitation spectrum of C12CS seeded in a supersonic beam of argon are shown
in figs. la and 2a. Figs.
lb and 2b depict
the spectra in the same regions recorded
without the argon carrier gas. The vibrational cooling achieved in the present experiments has greatly reduced the congestion in several regions of the spectrum, thus facilitating the analysis. Some of the assignments based on our measurements of peak positions, isotope shifts and on the relative intensities of spectral features with and without the argon carrier gas, are somewhat at variance with * According to the rotationalanalysis of Lombardi [3], only. ‘R branch forms a sharp peak and PP gives a weaker open structure (AK # Af lines are expected to be much weaker). The features listed in table 1 are therefore probably ‘R peaks.
LASER
WAVELENGTh/;
Fig. 1. (a) Excitation spectrum of Cl$Z3. seeded in argon, in the regions 19442-19430 cm-’ and 18720-18685 cm-‘. (IY) Excitation spectrum of Cl2CS without the argon carrier gas
251
Volume
76.
number 2
CHEMICAL
1 December 1980
PHYSICS LETTERS ments,
the upper state rotational
constants
calculated
by Lombardi [3] are probably those for the 12~3~ level. A curious feature of the cooled excitation spectrum is the strong peak at 19434.2 cm-l, which is not prominent in absorption and correlates with a shoulder in the unseeded thiophosgene excitation spectrum * , but is too intense to have contribution from only the wing of 2;3;4; of 35Ci37CICS. The only possible assignment seems to be 4;66. This assignment is, however. regarded
Fk. 2. (a) Portions of the excitation spectrum of QCS.
seed-
ed in argon, in the region 20350-19500
cm-‘. (b) Escitation spectrum of CIZCS without the argon carrier gas.
previous assignments. For example, the sharp peak in the excitation spectrum at 18715.8 cm-l (fi 1) is certainly a hot band (3:4;) and masks the 4.K-band which is located slightly to the red. The hot transition 263?4; is situated at 19195.5 (19192.6) cm-‘,where the number in parentheses refers to 35C137CICS. Transition 2:4: is reidentifed at 19194.8 (19192.6) cm-l. The feature at 19210.3 (19203.6) cm-’ is due to the hot band 2;364: and 364,$, to which this was previously attributed, is located at 19206.9 (19204.4) cm-l. The peak at 19395.6 (193909) cm-l is reassigned to 2;3:4; on the basis of isotope shift. In a similar way, the relative intensities in the excitation spectrum of seeded and unseeded thiophosgene pin down the
l
1;4:, which is exp ected in this region, is probably masked bymuch stronger transitions in the excitation spectrum.
-252
since we do not detect
pro-
* The possibility that this peak is due to a thiophosgeneargon van der Waals armpiex is ruled out since the intensity relative to neighboring features remains unchanged when the argon pressure is raised tenfold.
Table 2 The vibrational
19509.7 (19501.6) cm-l band as a hot transition, 2;3;4;, with 3$46, to which this was previously assigned, located at 19507.2 (19502.7) cm-L. An important reassignment is the band at 194409 (19435.0) cm-l which was allocated to 2;3;4; [l) and has been the subject of a rotational analysis [3] _ Since this peak disappears in the cooled excitation spectrum (see fig. 1), it is undoubtedly a hot band (263:4:). The 2;364: transition is identified at 19437.4 (19433 -0) cm-L l. In view of these reassign-
as tentative
gressions built on it. In any case, since the absorption spectrum does not reveal a strong transition in this position, there seems to be a state-dependent variation of fluorescence quantum yield in this region. The region near 5081 A includes the hot bands 223043 and 7340 m addition to the cold transitions -2 f’ already identified by Brand et al. 949 2,364, and 2040 [l] _To the blue of this region, more hot bands are detected at 19734.6 (19727.7), 19919.0 (19912.6), 20103.1 (20098.7), 20342.8 (20340.6) and 20347.7 (20342.8) cm-l. The high-frequency end of the excitation spectrum extends slightly to the blue of the assignments of Brand et al. The peaks in this region are included in table 1. Finally, vibrational frequencies,
vt (at) v2
(a11
v3
(al)
“4
@I)
“5 (b2) v6 (b2)
frequencies
in the excited and ground states
Ground state (‘AI)
Excited
=c1*cs
3sc12cs
35cl~7clCs
c) c)
502.9 288.9 470.8
state (‘AZ) =-Tl~7c1cs
906.7
905.6
479.9 246.2 V4= Lb) ~0
477.7 244.0 0
v4=3 v4=5
291 586 c)
2=90 582
367
365
a)
506.3 290.2 471.3
even
a) Too high to be excited at the temperature of OUTsample.
. b, Levels 2 and 4 are seen in our excitation spectrum only m combination with other modes. but b-these cases are perturbed 111. =’ TOO Weak to see; though not Formally Forbidden
Volume 76. number 2 calculated
from
some
CHEMICAL PHYSICS LE?TERS of
the separation
of peaks
listed
in table I, are collected in table 2. Fluorescence lifetime measurements of well-defied quantum states of the isolated molecule excited in a molecular beam would be very interesting. However, in the green and blue-green
regions of the excitation
spectrum, the excited state lifetime of = 35 PS exceeds the transit time of molecules across the field of fluorescence detection, making these measurements impossible. The lifetimes (< 150 ns) in the blue region are favorable but the signals in the present experiments were too weak. However, measurements in this region with an improved
detection
system should be
1 December 1980
References
[l] J-CD. Brand, J.H.CaUomon. DC. hloule. J.TyreUand T.H. Goodwin.Trans. Faraday Sot. 61 (1965) 2365.
[?I L. Burnelle. Acad. Roy. Bek.. CL Sci. Mem. 30 (1958)
No. 7. [3] J-R. Lombardi. J.Chem. Phys. 52 (1970) 6126. _ [4] J.R. hfd)onald and LE. Brus.Chem. Phys. Letters 16 (1972) 687. [S] J.C.D. Brand. J.L. Hardwick and K.-E. Tea. J. hfol. Spectry. 57 (1975) 215. 161 _ . J-R. Lombardi, J.R. Koffend. RA. Cottscho and R-IV_ Field, J. hIol. Spectry. 65 (1577) 446; P.R. Berruth, P.G. Cummins, J.R. Lombardiand R.W. Field, J. hfoL Spectry. 69 (1978) 166_
possible.
Acknowledgement This work was financially supported by grants from the National Science Foundation to Edward C. Lim and W. Martin McClain.
253
Volume 76, number 2
LASER-INDUCED COOLED
R. VASUDEV, Department
FLUORESCENCE
BY SUPERSONIC
Y. HIRATA,
of Chemistry.
Received 39 July 1980;
PHYSICS
EXClTATION
1 December
LETTERS
SPECTRUM
1980
OF THIOPHOSGENE
EXPANSION
EC. LIhl and WM. M&WIN
Wayne State University. Detroit, Michigan 182017. US.4
in final iorm 22 August 1980
The vIbrational structure in the laser-induced fluorescence excitation spectrum of the fust excited TT*.- n transition of thiophogene, seeded in a supersonic beam of argon, is reported. Hot bands are identified empirically by variation of beam temperature, and vibronic assignments are adjusted as required.
1. Introduction
2. Experimental
The visible x ‘A,-R ‘A, transition of thiophosgene, CIZCS, has been studied by many workers [l-3]. In the first comprehensive work, Brand et al. [I] established the vibrational analysis in considerable detail and identified the vibrational frequencies in the excited and ground states of 35CIZCS and 35C137ClCS_ FranckCondon analysis was used IO arrive at possible excited state structures. The high-resolution spectrum has resisted detailed rotational analysis due to strong overlap of bands of the two isotopic species and, in several regions, of hot and cold bands. In the only rotational analysis work to date, Lombardi [3] performed a contour analysis of a band near 5 142 A which was assigned by Brand et al. [l] to the 2;3;4; band of 35C1~CS. In this paper, we report the laser-induced fluorescence excitation spectrum of thiophosgene seeded in a supersonic beam of argon. The vibrational structure is fairly well resolved_ We have detected some hitherto unreported transitions and have empirically identified several hot bands by varying the vibrational temperature. These features have been reassigned as required. Laser-excited fluorescence in the isolated molecule has also been studied by Brus and McDonald 141, Brand et al. [S], and Lombardi et al. [6]. The results of Brus and McDonald imply state-independent fluorescence lifetime and quantum yield to the red of 4550 A (where photodissociation sets in). In one region of our excitation spectrum we notice, on the contrary, a statedependent quantum yield variation_
ClzCS (at its room temperature vapor pressure) was mixed with argon and expanded through a 100~ nozzle. the stagnation pressure of the argon being = 0.1-0.15 atm. The molecular jet was crossed with an unfocused pulsed laser beam from a Quanta-Ray model PDL-1 dye laser which was pumped by the third harmonic of a Nd : Yag laser operating at 10 Hz. The 03 cm-t fwhm, 5 ns, 75-500 kW peak power pulsed beam passed through a series of light baffles and intersected the molecuiar beam 5- 10 mm downstream from the nozzle. The fluorescence was filtered by sharp cut-off filters, detected by an RCA C7 15 IW photomultiplier, processed by a boxcar averaget and then recorded on ay-t recorder. A wavelength caIibration was established by recording strong features in the absorption spectrum of thiophosgene at room temperature with a very small portion of the Iaser beam. These were then compared with the published peak positions [ 1,2] _The measurements in these regions should be accurate to 0.5 cm-l. The positions of peaks in other regions of the escitation spectrum were then obtained by interpolation. Small portions of the excitation spectrum in the region 1868G-20350 cm-l were also recorded without the argon carrier gas. These scans involve higher vibrational temperatures and were invaluable in the identification of several spectra1 features as hot bands.
details
249
CHEMICAL
Volume 76. number2
PHYSICS
1 December 1980
LETTERS
Table 1 Excitation spectrum of thiophosgene a)
;;(cnl-‘)
iqcm-’ )
Assignment 35Cl+zS
Assignment 35Cl~CS
3%Y’CKS
ii(cm-’ ) %Y’cLcs
35c12cs
2:4;
19204.4 19206 9
3204;
19669.3
18690.9
4’:
19210.3
2;3;4:
19673.0
18706.0
4;6:
1a252.5
18688.5 18689.7
18708.0
19256.5
18714.9
46
19297.3
18715.8
3O43 I 0
19301.1
18729.7
19641.9 (c)
19674.1 31430 5 4,
19323.1
18731.8
3204:
19326.7
18785 (v.br)
3;4: b)
193909
Assignment
3;6:,
19675.7
2; 3:4;
19679.0
2847
19680.8
2;4:,
19684.1
2i3;4:,
197265 19727.7
2;3:4:,
19730.9
18849.2
19395.6
18894.8
19430.5
19733.2
2;3;41
18900.1
19433.0 (sh)
19734.6
2:3;4:
19434.2
19805.8 (c)
19435.0
19807.8 (c)
19437.4
19853.1
19004.7 (sh)
194409
19856.1
19006.0
19451.9
19868.6
18902.3
2; 3y4:,
18958.9 18961.1
3646
4:
19156.7
19871.1
32047
19476.2
19912.6
19481.1
19913.9
6;
19501.6
19916.0
19095.7 (c)
19502.7
19917.0 (h)
19097.8 (c)
19507.2
19919.9
19035.2 19038.9 19080.1 19081.9
19143.2 (sh) 191451
2$3?4!,
19148.8 19149.7 (sh) 19150.3
1;4y
19 162 (br)
19509.7
19922 (al)
19551.8
19927.0
19555.2
19930.2
19572.3
19983.1
19576.7
19985.0
3;4;6:,
1;3:4;
1;4; 310 203041
2;3;4:,
1;6;
19605.5
19190.0
19607.4
%3?4&2h4k
19192.6
19608.6
20098.7
19194.8
2;4:,
19620.5
19195.5
2;3:4;
19621.6
20100.6
1;2:4;
19636.4 (c)
2OlCj3.1
1;2;4:
19640.0 (c)
20111.4 (c)
19196.7 19203.6 250
2;3;4:
3V13’cK3s
Volume 76, number 2
CHEMICAL
PHYSICS
L December
LE-fTERS
1980
Table 1 (cant) I(cm-‘)
Assignment ssc12cs
20113.3 20116.9
1;3;4:,
2:3;4:,
20176.1
3scl~‘clCs 1;2;3;4:
20342.8
1204:
20345.2
1;2;3h4;
20317.7
1;2;3;4;
203040 1 4 I
20416.0
lb32041
i0516.1
1’04;b)
1:3x4;
20209.5
20579.7
20223.7
20582.3 (sh) 20590.4 1;2;3;4:,
20228.4 20229.7 20234.7
1;3;6:,
20765.2
20284.5 (c) 20287.0 (c) 20340.6
20822.7 20836.7 1
1541. z l;-:3&4?) -.
20998.4
21239.1
21280.6 212865
71313.0
1;2:4;
21320.9
1;2;3:4:,
21478.1
1;2;4:,
21472.2 21481.2
1$3A4:,
208 16.8
1;3;6h
3sc13’cIcs
21214.1
20409.8
20208.1
Wl2CS
21069.7
1;2;4;
20572.8
Assignment
21063.4 1;2;4;
20386.8
7’ 343’ -0 0 0
S(cm_’ )
3sc1zcs
1;3;4;
1;3;43,
20169 (sh)
Assignment ~____-
20381.8
2; 3;4:,
20164.6 20166.8
3scP’clCs
1;3;4:,
20153.2 20160.0
5(cm-*)
1;2;3;4:, 1:2;3;4; 1;2;3;4;
20994.2
21489.1 21536.8 21709.9
b) 152646
21717.9
a1 br = broad, sh = shoulder, c = cold band, h = hot band. b, The corresponding band of 35~37C~CS is hidden or below the noise level_
3. Results and discussion Table 1 presents the measured peak positions and their assignments *. Portions of the excitation spectrum of C12CS seeded in a supersonic beam of argon are shown
in figs. la and 2a. Figs.
lb and 2b depict
the spectra in the same regions recorded
without the argon carrier gas. The vibrational cooling achieved in the present experiments has greatly reduced the congestion in several regions of the spectrum, thus facilitating the analysis. Some of the assignments based on our measurements of peak positions, isotope shifts and on the relative intensities of spectral features with and without the argon carrier gas, are somewhat at variance with * According to the rotationalanalysis of Lombardi [3], only. ‘R branch forms a sharp peak and PP gives a weaker open structure (AK # Af lines are expected to be much weaker). The features listed in table 1 are therefore probably ‘R peaks.
LASER
WAVELENGTh/;
Fig. 1. (a) Excitation spectrum of Cl$Z3. seeded in argon, in the regions 19442-19430 cm-’ and 18720-18685 cm-‘. (IY) Excitation spectrum of Cl2CS without the argon carrier gas
251
Volume
76.
number 2
CHEMICAL
1 December 1980
PHYSICS LETTERS ments,
the upper state rotational
constants
calculated
by Lombardi [3] are probably those for the 12~3~ level. A curious feature of the cooled excitation spectrum is the strong peak at 19434.2 cm-l, which is not prominent in absorption and correlates with a shoulder in the unseeded thiophosgene excitation spectrum * , but is too intense to have contribution from only the wing of 2;3;4; of 35Ci37CICS. The only possible assignment seems to be 4;66. This assignment is, however. regarded
Fk. 2. (a) Portions of the excitation spectrum of QCS.
seed-
ed in argon, in the region 20350-19500
cm-‘. (b) Escitation spectrum of CIZCS without the argon carrier gas.
previous assignments. For example, the sharp peak in the excitation spectrum at 18715.8 cm-l (fi 1) is certainly a hot band (3:4;) and masks the 4.K-band which is located slightly to the red. The hot transition 263?4; is situated at 19195.5 (19192.6) cm-‘,where the number in parentheses refers to 35C137CICS. Transition 2:4: is reidentifed at 19194.8 (19192.6) cm-l. The feature at 19210.3 (19203.6) cm-’ is due to the hot band 2;364: and 364,$, to which this was previously attributed, is located at 19206.9 (19204.4) cm-l. The peak at 19395.6 (193909) cm-l is reassigned to 2;3:4; on the basis of isotope shift. In a similar way, the relative intensities in the excitation spectrum of seeded and unseeded thiophosgene pin down the
l
1;4:, which is exp ected in this region, is probably masked bymuch stronger transitions in the excitation spectrum.
-252
since we do not detect
pro-
* The possibility that this peak is due to a thiophosgeneargon van der Waals armpiex is ruled out since the intensity relative to neighboring features remains unchanged when the argon pressure is raised tenfold.
Table 2 The vibrational
19509.7 (19501.6) cm-l band as a hot transition, 2;3;4;, with 3$46, to which this was previously assigned, located at 19507.2 (19502.7) cm-L. An important reassignment is the band at 194409 (19435.0) cm-l which was allocated to 2;3;4; [l) and has been the subject of a rotational analysis [3] _ Since this peak disappears in the cooled excitation spectrum (see fig. 1), it is undoubtedly a hot band (263:4:). The 2;364: transition is identified at 19437.4 (19433 -0) cm-L l. In view of these reassign-
as tentative
gressions built on it. In any case, since the absorption spectrum does not reveal a strong transition in this position, there seems to be a state-dependent variation of fluorescence quantum yield in this region. The region near 5081 A includes the hot bands 223043 and 7340 m addition to the cold transitions -2 f’ already identified by Brand et al. 949 2,364, and 2040 [l] _To the blue of this region, more hot bands are detected at 19734.6 (19727.7), 19919.0 (19912.6), 20103.1 (20098.7), 20342.8 (20340.6) and 20347.7 (20342.8) cm-l. The high-frequency end of the excitation spectrum extends slightly to the blue of the assignments of Brand et al. The peaks in this region are included in table 1. Finally, vibrational frequencies,
vt (at) v2
(a11
v3
(al)
“4
@I)
“5 (b2) v6 (b2)
frequencies
in the excited and ground states
Ground state (‘AI)
Excited
=c1*cs
3sc12cs
35cl~7clCs
c) c)
502.9 288.9 470.8
state (‘AZ) =-Tl~7c1cs
906.7
905.6
479.9 246.2 V4= Lb) ~0
477.7 244.0 0
v4=3 v4=5
291 586 c)
2=90 582
367
365
a)
506.3 290.2 471.3
even
a) Too high to be excited at the temperature of OUTsample.
. b, Levels 2 and 4 are seen in our excitation spectrum only m combination with other modes. but b-these cases are perturbed 111. =’ TOO Weak to see; though not Formally Forbidden
Volume 76. number 2 calculated
from
some
CHEMICAL PHYSICS LE?TERS of
the separation
of peaks
listed
in table I, are collected in table 2. Fluorescence lifetime measurements of well-defied quantum states of the isolated molecule excited in a molecular beam would be very interesting. However, in the green and blue-green
regions of the excitation
spectrum, the excited state lifetime of = 35 PS exceeds the transit time of molecules across the field of fluorescence detection, making these measurements impossible. The lifetimes (< 150 ns) in the blue region are favorable but the signals in the present experiments were too weak. However, measurements in this region with an improved
detection
system should be
1 December 1980
References
[l] J-CD. Brand, J.H.CaUomon. DC. hloule. J.TyreUand T.H. Goodwin.Trans. Faraday Sot. 61 (1965) 2365.
[?I L. Burnelle. Acad. Roy. Bek.. CL Sci. Mem. 30 (1958)
No. 7. [3] J-R. Lombardi. J.Chem. Phys. 52 (1970) 6126. _ [4] J.R. hfd)onald and LE. Brus.Chem. Phys. Letters 16 (1972) 687. [S] J.C.D. Brand. J.L. Hardwick and K.-E. Tea. J. hfol. Spectry. 57 (1975) 215. 161 _ . J-R. Lombardi, J.R. Koffend. RA. Cottscho and R-IV_ Field, J. hIol. Spectry. 65 (1577) 446; P.R. Berruth, P.G. Cummins, J.R. Lombardiand R.W. Field, J. hfoL Spectry. 69 (1978) 166_
possible.
Acknowledgement This work was financially supported by grants from the National Science Foundation to Edward C. Lim and W. Martin McClain.
253