- Email: [email protected]
Dispersed phosphorescence spectra in a supersonic free jet by electric discharge excitation
22 November 1996
CHEMICAL PHYSICS LETTERS ELSEVIER
Chemical Physics Letters 262 (1996) 615-620
Dispersed phosphorescence spectra in a supersonic free jet by electric discharge excitation Yoshinori Nibu, Daisuke Sakamoto, Takeshi Satho, Hiroko Shimada Department of Chemistry. Faculty of Science, Fukuoka University. Nanakuma. Jyonan-ku, Fukuoka 814-80, Japan
Received 22 May 1996; in final form 5 August 1996
Abstract
Pyrazine and benzophenone were excited into the lowest triplet state by electric discharge and their dispersed phosphorescence spectra in a supersonic free jet were observed. The bandwidth of the spectrum of pyrazine is narrower than that of the spectra obtained with laser excitation. The experiment shows that electric discharge excitation is an efficient technique for creating a triplet state molecule under collision free conditions.
I. Introduction
Information on the potential surface of the triplet state of organic molecules gives a clue to the study of the photophysical and photochemical behavior of electronically excited molecules. Especially, experiments on collision free molecules provide an ideal condition for the investigation of the intrinsic nature of the molecule because of the absence of disturbance from surrounding molecules. The potential surface of the excited singlet state of a collision free molecule has been well established for many molecules through recent progress in laser spectroscopy and the supersonic jet technique [1]. However, the potential surface of the triplet state was hard to determine because of the weak transition probability of the excitation from the ground to triplet states due to differences in the spin multiplicity. In order to determine the potential surface of the
excited state, analysis of both the absorption and emission spectra are needed. The former determines the levels in the excited state accessed from the zero point vibrational level in the ground state, and the latter determines the vibrational levels in the ground state accessed from the electronically excited levels. For the singlet state, the observation of absorption (fluorescence excitation) and emission (dispersed fluorescence) spectra in supersonic free jets is not so difficult except for non-emitting molecules, such as pyridine. The observation of singlet-triplet transitions of pyrazine and benzophenone in a supersonic free jet could be made if high sensitive spectroscopic techniques such as phosphorescence excitation [213], sensitized phosphorescence excitation [14], resonantly enhanced multi-photon ionization (REMPI) [13,15] and surface ejection of electron by laser excited metastables (SEELEM) [11], are used. However, observation of the dispersed phosphorescence
0009-2614/96/$12.00 Copyright © 1996 Elsevier Science B.V. All rights reserved PII S0009-2614(96)01 124-4
616
Y. Nibu et al. / Chemical Physics Letters 262 (1996) 615-620
spectrum in supersonic free jets is quite difficult because the molecule in the lowest triplet state has a long lifetime, at least longer than a few ms, and thus the triplet molecule runs away from the focal point of the collimator lens of the spectrometer. The off focus moving of the emitting molecule markedly decreases the efficiency of collimation of phosphorescence and makes the observation of the spectrum difficult. However, in spite of the disadvantage, dispersed phosphorescence spectra of pyrazine in jets have been observed by Penner and Amirav [12] and Ottinger and Vilesov [10]. In the experiment of Penner and Amirav, pyrazine was excited directly into the lowest triplet state with a tunable dye laser in a supersonic jet expanding through a slit nozzle and the spectrum was measured with a resolution of 1 nm. Recently, Ottinger and Vilezov also observed the dispersed phosphorescence spectrum in a jet with a resolution of 0.2 nm. For benzophenone, no dispersed phosphorescence spectrum has been observed in a jet. The electric discharge technique is an effective method to exciting a molecule into optically forbidden excited states, such as the triplet state, from the ground state. The triplet molecules can be created by energy transfer through collision with excited molecules, electrons, ions and excited carrier gas molecules. Sharp and Johnson [16] detected the lowest triplet state of benzene with this method. In their experiment, benzene was excited into the lowest triplet state through electric discharge in a supersonic free jet and the photoionization spectrum was measured by ionization from the triplet state with a tunable UV laser. A few studies have been made for the observation of the phosphorescence spectrum of pyrazine in the vapor phase. Ishii et al. [17] observed the spectrum by ultraviolet irradiation with a high pressure mercury lamp. Inoue and Ebara [18] also observed the spectrum through electron impact excitation under low vapor pressure. However, the spectra observed with these methods did not show well resolved vibrational fine structure because of the higher vibrational and rotational energy distribution in molecules at room temperature. In this Letter, the dispersed phosphorescence spectra of pyrazine and benzophenone in a supersonic free jet without laser excitation are shown and
the advantage of the electric discharge excitation of organic molecules into the lowest triplet state in a supersonic free jet is discussed.
2. Experimental The experimental apparatus used for the electric discharge excitation of the sample is shown in Fig. 1. The pulsed jet nozzle of a fuel injector of an automobile [19] was modified for our purpose and a pair of electrodes made of thin brass (1 mm thickness) plates were attached to the top of the nozzle exit. Holes for the expansion in a jet were made in the centers of the electrodes fixed by insulators. Diameters of the holes of the electrodes and insulators were determined to give the lowest rotational and vibrational temperatures of the expanding molecules, by observing the fluorescence excitation spectra of the parent molecules, such as pyrazine. The vacuum chamber for the jet was evacuated by a 6 in. diffusion pump backed by an oil rotary pump. The operation of the pulsed jet nozzle was performed by our home-made nozzle driver. The nozzle driver also triggers a high voltage pulse power supply, Matsusada Precision Devices HJK-2P50-FU. The electric pulse of 2000 V, 50 /xs width was delayed from the electric pulse for opening the nozzle so that the glow discharge on the electrodes was synchronized with the expansion of molecules from the nozzle. The discharge creates metastable molecules, ions and free radicals. The triplet state molecules expanding into the vacuum chamber were effectively cooled through collision with a certain carrier gas. The pressure of the carrier gas was kept as low as possible because the high pressure of the gas considerably increases the emission from the metastable states of the carrier gas molecule comparing to the phosphorescence intensity of the molecule under study. The emission induced by the discharge was collimated with a large aperture quartz lens. The dispersed emission was detected with a Spex 1269 monochromator equipped with a photomultiplier tube of EMI-9789QB and the signal was processed with a gated integrator of Stanford Research Systems SRS250. Several kinds of carrier gas such as He, Ar and N 2 were used for the observation of the spectrum.
Y. Nibu et al./ Chemical Physics Letters 262 (1996) 615-620
Carriergas
617
[ NozzleDriver I
HighVoltage PowerSupply HJK-2P50-FU 2000V
SPEX-1269
\
SiC.View
Fq IR''r" I
Insulator ~ l ~ e~ t ct ~ t ~rt \°Nozzle I d E]~0Ite Front s View \ Electrodes'~~~~
+ ~ ~
Fig. 1. Apparatus used for the observation of the discharge phosphorescence spectrum in a supersonic free jet.
The bands due to the phosphorescence should be commonly observed for all the spectra with different kinds of carrier gas. Pyrazine and benzophenone from Wako were used without further purification.
3. Results and discussion
3.1. Dispersed phosphorescence spectrum of pyrazine Fig. 2 shows the dispersed emission spectra of pyrazine obtained by electric discharge in a super-
sonic free jet with He carrier gas under different observing conditions. The bands due to the metastable state of the carrier gas were picked up using the MIT wavelength table [21]. These bands were used for calibration of the bands of pyrazine phosphorescence. A strong band at 372.75 nm exactly coincides with the 0 - 0 band of the T~-S 0 transition of pyrazine observed in the gas phase in earlier works [5,6,11-14,15,17,18]. The spectrum emitted near the exit of the nozzle is shown in Fig. 2a. Extremely strong bands were observed around 387.5 nm. Through the observation of the high resolution spec-
618
Y. Nibu et aL / Chemical Physics Letters 262 (1996) 615-620
420
410
b) 0
420
410
400
_ 0 0 lOa 0 ~a 16a I
4 0 0 Llnm
390
9a 1o ©~0 /v~2
390
380
370
to~ 0
380
370
Fig. 2. Dispersed emission spectra of pyrazine observed by electric discharge excitation in a supersonic free jet with He. (a) Spectrum of the emission from the center of the nozzle exit and (b) spectrum of the emission far from the nozzle exit. The bands marked with * are due to the emission from metastable He.
trum, these bands were assigned to the rotational structure of the band origin of the transition from the B(22~ +, v' = 0) to X(2• +, u n = 0) states of the CN radical [20], which is one of the fragment species of pyrazine decomposed by the electric discharge. The same band structures were observed in the discharge spectra of CH3CN and pyrazine-d 4. These observations suggest that pyrazine is decomposed by electric discharge and the electronically excited CN radical is produced in a jet. The strong bands due to the CN radical and He overlap with the vibronic bands of the phosphorescence spectrum of pyrazine especially around 387.5 nm region as shown in Fig. 2a. Fig. 2b shows the spectrum observed the discharge emission from the region far from the center of the nozzle exit. The intensity of the bands due to the excited CN radical and He decreases drastically, but the intensity of the bands due to pyrazine phosphorescence does not decrease compared with those of CN and He. This observation can be explained as the difference in the lifetimes of the emitting species. The lifetime of the CN radical in the B state, 67.7 ns [22], is so short that the emission from the CN radical can only be observed near the center of the nozzle exit, where pyrazine is excited and the CN radical is created by the electric discharge. On the other hand, the lifetime of the lowest triplet state of
pyrazine is 4.45 ms [11] in a supersonic free jet and the triplet pyrazine can survive even in the region far from the nozzle exit. The observation was made by moving the collimator lens horizontally so that the emitting portion of the off-center jet was focused onto the slit of the monochromator. The 'space resolved spectrum technique' could be used for the effective selection of the phosphorescence of the molecule under study among emissions from many other species. The dispersed phosphorescence spectrum of pyrazine excited by the electric discharge in the jet gives higher resolution than those observed in earlier works made in a supersonic free jet. For example, a single band at 1830 cm-~ observed in the spectra of the previous works [10,12] splits into doubles, 1828 and 1840 c m - ~, in the present work. The former band is assigned to the 9a~6a] vibrations and the latter to the 10a 2 vibration. The bands due to the 9a t and 6a 2 vibrations are also clearly resolved, but were not resolved in the earlier works. The full-width at half-maximum (FWHM) of the 0 - 0 band of the discharge excited phosphorescence spectrum of pyrazine was about 0.15 nm and the vibronic structure was the same as those observed in the gas and cold matrix phases [23]. These observations indicate that the spectral structure of the pyrazine in both collision free and cold matrix phases has no intrinsic difference.
3.2. Phosphorescence spectrum of benzophenone The dispersed phosphorescence spectrum of benzophenone excited by electric discharge with He carrier gas is shown in Fig. 3. As the lifetime of the lowest triplet state of benzophenone is considered to be shorter than that of pyrazine judging from the lifetimes observed in a cold matrix, the phosphorescence intensity decreases sensitively by the change of the observing region by moving the collimator lens. The origin band of the discharge excited emission spectrum of benzophenone was observed at 412.92 nm, where the 0 - 0 band of the sensitized phosphorescence excitation spectrum of benzophenone was observed by Ohmori et al. [14] in a supersonic free jet. As can be seen in the figure, a low frequency progression spaced by 62 c m - i was observed. The
Y. N ibu et al. / Chemical Physics Letters 262 (1996) 615-620
s,i2 60
I
I
I
I
,I 420
1
I
2
i
o: 4 1 2 . 9 2 n m
60
6;
II
i
,
,
~./nm
c m "1
410
Fig. 3. Dispersed phosphorescence spectra of benzophenone in the 0 - 0 band region observed in a supersonic free jet. The bands marked with * are due to the emission from metastable He.
progression shows the mirror image relation with the progression observed in the phosphorescence excitation spectrum of benzophenone. These low frequency bands and the bands with 1700 c m - t spacings, which could be assigned to the C = O stretching vibration, form the main progression of the phosphorescence spectrum. These observations definitely give proof that the observed spectrum is the phosphorescence spectrum of benzophenone. The frequency of 62 c m - ' was assigned to the frequency of the totally symmetric torsional vibration of the two benzene rings in the ground state. The normal coordinate calculation shows that the frequency of the torsional vibration, whose mode is symmetric with respect to the C2(C=O) axis, is 57 cm -1 in the ground state [24]. The corresponding Raman band was observed as a weak shoulder band on the Rayleigh wing of benzophenone dissolved in benzene [24]. Seven quanta of the torsional vibration with constant spacing were observed in the low frequency progression. This observation indicates that the torsional vibration is strictly harmonic up to the higher vibrational levels. The same is also true for the vibration in the S I or T~ states [14,25-27]. The progression in the spectrum arises from the difference in the molecular structure between the ground and excited states along the symmetric torsional normal coordinate. Frederick et al. [25] and Kamei et al. [27] have made a Franck-Condon analysis of the vibronic structure of their spectra. They showed that the torsional angle of
619
each phenyl ring in the S I state decreases about 13° from that in the ground state due to the electric excitation. In their calculations, the frequency of 57 c m - l was used as the ground state frequency of the symmetric torsional vibration. The value of our experiment slightly changes the value of the calculated torsional displacement. The frequencies of the symmetric torsional vibrations of benzophenone in the S 0, S~ and T~ states are 62, 63 and 68 cm - f , respectively. The frequency of the torsional vibration could be affected by the strength of the 7r conjugation of the two phenyl rings through the carbonyl group. Thus the zr conjugation in the T~ state is stronger than the conjugation in the S O and S~ states. On the other hand, the FranckCondon maximum of the low frequency progression in the S j-S 0 transition is located at higher vibrational levels compared with that in the T~-S 0 transition. This fact suggests that the displacement of the torsional angle of the two phenyl rings from the equilibrium structure caused by the S ~-S 0 transition is greater than the displacement caused by the T~-S 0 transition. The excitation of the localized non-bonding electron into the delocalized 7r * orbital strengthens the conjugation of the two benzene rings. Therefore, the molecular structure in the excited states is closer to planar than in the ground state. The Franck-Condon analysis of the vibronic structure indicates that the degree of planarity in the electronic states is S~ > T, > S O. From the point of view of frequency, the order is T~ > S~ ~ S 0. The reason for the difference is now in question and the discrepancy of the results may be solved by a precise molecular orbital calculation of the excited states. The dispersed phosphorescence spectrum of benzophenone in a jet shows a considerably different structure from that observed in a cold solid matrix [28]. In the spectrum observed in 4,4'-dibromodiphenyl ether host crystal at 1.4 K, the intensity of the progression due to the symmetric torsional vibration having a frequency of 103 cm -~ was weak, although the intensity of the 0 - 0 band was strongest. This observation indicates that the molecular structure of benzophenone excited into the lowest triplet state in a cold matrix is different from that in a jet. Benzophenone has a flexible molecular structure for the torsion of the phenyl rings around the C(carbonyl)-C(phenyl) axis, and the structure is eas-
620
Y. Nihu et a L / Chemical Physics Letters 262 (1996) 615-620
ily affected by the perturbation from the surrounding molecules. The molecular structure of free benzophenone is considered to be n o n - p l a n a r in the S O state and more planar in the S 1 and T~ states [14,25-27]. Although the molecular structure in the jet is governed only by the intramolecular potential, the molecular structure in a cold matrix is governed by the balance of the inter- and intramolecular interactions. The spectral structure of the phosphorescence observed in a cold matrix suggests that the T ~ - S 0 excitation does not induce a considerable change in the molecular structure. This fact indicates that the molecular structure of b e n z o p h e n o n e is quite sensitive to the environment. In other words, the molecular structure of b e n z o p h e n o n e in the c o n d e n s e d phase is affected by the surrounding molecules. Therefore, the study of the solvation effect on the electronic spectrum of b e n z o p h e n o n e is expected to give information on the microscopic e n v i r o n m e n t a l effect on the molecular structure.
4. Conclusion The dispersed phosphorescence spectra of pyrazine and b e n z o p h e n o n e excited by electric discharge in a supersonic free jet show a highly resolved vibronic structure. The discharge emission spectroscopy would provide a useful method for studying the behavior of the lowest triplet molecules under collision free conditions. This technique is useful for the observation of low frequency vibronic structure such as internal methyl rotation. This method would be applicable to the observation of the emissions of metastable molecular species, such as free radicals or molecular ions. The application of this technique to metastable molecules will appear soon.
Acknowledgements This work was supported in part b y a G r a n t - i n - A i d for the E n c o u r a g e m e n t of Y o u n g Scientists by the Ministry of Education, Science and Culture.
References [1] See, for example, J.M. Hollas and D. Phillips, Jet spectroscopy and molecular dynamics (Blackie Academic & Professional, Glasgow, 1995), and references therein. [2] L.H. Spangler, Y. Matsumoto and D.W. Pratt, J. Phys. Chem. 87 (1983) 478 I. [3] L.H. Spangler and D.W. Pratt, J. Chem. Phys. 84 (1986) 4789. [4] L.H. Spangler, D.W. Pratt and F.W. Birss, J. Chem. Phys. 85 (1986) 3229. [5] A. Penner, Y. Oreg, E. Villa, E.C. Lira and A. Amirav, Chem. Phys. Lett. 150 (1988) 243. [6] J.L. Tomer, L.H. Spangler, K.W. Holtzclaw and D.W. Pratt, J. Chem. Phys. 88 (1988) 1528. [7] J.L. Tomer, L.H. Spangler and D. Pratt, J. Am. Chem. Soc. I10 (1988) 1615. [8] K.W. Holtzclaw, L.H. Spangler and D.W. Pratt, Chem. Phys. Lett. 161 (1989)347. [9] J.L. Tomer, S. Yamauchi and D.W. Pratt, Chem. Phys. Lett. 175 (1990) 3O. [10] C. Ottinger and A.F. Vilesov, Z. Physik. Chem. 188 (1995) Ill. [I 1] O. Sneh, D. Diinn-Kittenplon and O. Cheshnovsky, J. Chem. Phys. 91 (1989) 7331. [12] A. Penner and A. Amirav, J. Chem. Phys. 92 (1988) 5079. [13] E. Villa, A. Amirav, W. Chen and E.C. Lim, Chem. Phys. Lett. 147 (1988) 43. [14] N. Ohmori, T. Suzuki and M. lto, J. Phys. Chem. 92 (1988) 1086. [15] E. Villa, M. Terazima and E.C. Lim, Chem. Phys. Lett. 129(4) (1986) 336. [16] S. Sharpe and P. Johnson, J. Chem. Phys. 81 (1984) 4176. [17] H. lshii, M. Koyanagi and Y. Kanda, Bull. Chem. Soc. Jpn. 44 (1971) 1205. [18] I. lnoue and N. Ebara, J. Phys. Chem. 89 (1985) 4654. [19] N. Mikami, A. Hiraya, A. Fujiwara and M. Ito, Chem. Phys. Lett. 74 (1980) 531. [20] W.M. Jackson, J. Chem. Phys. 61 (1974) 4177. [21] F.M. Phelps III, M.I.T. Wavelength Tables, Vol. 2: Wavelengths by Element (The MIT Press). [22] C. Anastasi and D. Hancock. J. Chem. Soc. Faraday Trans. 2 84 (1988) 9. [23] F. Kokai and T. Azumi, J. Chem. Phys. 75 (1981) 1069. [24] J. Blazevic and L. Colombo. J. Raman Spectrosc. 11 (1981) 143. [25] J. Frederick E. Heller J. Ozment and D. Pratt, J. Chem. Phys. 88 (1988) 2169. [26] K.W. Holtzclaw and D.W. Pratt, J. Chem. Phys. 84 (1986) 4713. [27] S. Kamei, T. Sato, N. Mikami and M. ito, J. Phys. Chem. 90 (1986) 5615. [28] S. Yamauchi and D.W. Pratt, Mol. Phys. 37 (1979) 541.
CHEMICAL PHYSICS LETTERS ELSEVIER
Chemical Physics Letters 262 (1996) 615-620
Dispersed phosphorescence spectra in a supersonic free jet by electric discharge excitation Yoshinori Nibu, Daisuke Sakamoto, Takeshi Satho, Hiroko Shimada Department of Chemistry. Faculty of Science, Fukuoka University. Nanakuma. Jyonan-ku, Fukuoka 814-80, Japan
Received 22 May 1996; in final form 5 August 1996
Abstract
Pyrazine and benzophenone were excited into the lowest triplet state by electric discharge and their dispersed phosphorescence spectra in a supersonic free jet were observed. The bandwidth of the spectrum of pyrazine is narrower than that of the spectra obtained with laser excitation. The experiment shows that electric discharge excitation is an efficient technique for creating a triplet state molecule under collision free conditions.
I. Introduction
Information on the potential surface of the triplet state of organic molecules gives a clue to the study of the photophysical and photochemical behavior of electronically excited molecules. Especially, experiments on collision free molecules provide an ideal condition for the investigation of the intrinsic nature of the molecule because of the absence of disturbance from surrounding molecules. The potential surface of the excited singlet state of a collision free molecule has been well established for many molecules through recent progress in laser spectroscopy and the supersonic jet technique [1]. However, the potential surface of the triplet state was hard to determine because of the weak transition probability of the excitation from the ground to triplet states due to differences in the spin multiplicity. In order to determine the potential surface of the
excited state, analysis of both the absorption and emission spectra are needed. The former determines the levels in the excited state accessed from the zero point vibrational level in the ground state, and the latter determines the vibrational levels in the ground state accessed from the electronically excited levels. For the singlet state, the observation of absorption (fluorescence excitation) and emission (dispersed fluorescence) spectra in supersonic free jets is not so difficult except for non-emitting molecules, such as pyridine. The observation of singlet-triplet transitions of pyrazine and benzophenone in a supersonic free jet could be made if high sensitive spectroscopic techniques such as phosphorescence excitation [213], sensitized phosphorescence excitation [14], resonantly enhanced multi-photon ionization (REMPI) [13,15] and surface ejection of electron by laser excited metastables (SEELEM) [11], are used. However, observation of the dispersed phosphorescence
0009-2614/96/$12.00 Copyright © 1996 Elsevier Science B.V. All rights reserved PII S0009-2614(96)01 124-4
616
Y. Nibu et al. / Chemical Physics Letters 262 (1996) 615-620
spectrum in supersonic free jets is quite difficult because the molecule in the lowest triplet state has a long lifetime, at least longer than a few ms, and thus the triplet molecule runs away from the focal point of the collimator lens of the spectrometer. The off focus moving of the emitting molecule markedly decreases the efficiency of collimation of phosphorescence and makes the observation of the spectrum difficult. However, in spite of the disadvantage, dispersed phosphorescence spectra of pyrazine in jets have been observed by Penner and Amirav [12] and Ottinger and Vilesov [10]. In the experiment of Penner and Amirav, pyrazine was excited directly into the lowest triplet state with a tunable dye laser in a supersonic jet expanding through a slit nozzle and the spectrum was measured with a resolution of 1 nm. Recently, Ottinger and Vilezov also observed the dispersed phosphorescence spectrum in a jet with a resolution of 0.2 nm. For benzophenone, no dispersed phosphorescence spectrum has been observed in a jet. The electric discharge technique is an effective method to exciting a molecule into optically forbidden excited states, such as the triplet state, from the ground state. The triplet molecules can be created by energy transfer through collision with excited molecules, electrons, ions and excited carrier gas molecules. Sharp and Johnson [16] detected the lowest triplet state of benzene with this method. In their experiment, benzene was excited into the lowest triplet state through electric discharge in a supersonic free jet and the photoionization spectrum was measured by ionization from the triplet state with a tunable UV laser. A few studies have been made for the observation of the phosphorescence spectrum of pyrazine in the vapor phase. Ishii et al. [17] observed the spectrum by ultraviolet irradiation with a high pressure mercury lamp. Inoue and Ebara [18] also observed the spectrum through electron impact excitation under low vapor pressure. However, the spectra observed with these methods did not show well resolved vibrational fine structure because of the higher vibrational and rotational energy distribution in molecules at room temperature. In this Letter, the dispersed phosphorescence spectra of pyrazine and benzophenone in a supersonic free jet without laser excitation are shown and
the advantage of the electric discharge excitation of organic molecules into the lowest triplet state in a supersonic free jet is discussed.
2. Experimental The experimental apparatus used for the electric discharge excitation of the sample is shown in Fig. 1. The pulsed jet nozzle of a fuel injector of an automobile [19] was modified for our purpose and a pair of electrodes made of thin brass (1 mm thickness) plates were attached to the top of the nozzle exit. Holes for the expansion in a jet were made in the centers of the electrodes fixed by insulators. Diameters of the holes of the electrodes and insulators were determined to give the lowest rotational and vibrational temperatures of the expanding molecules, by observing the fluorescence excitation spectra of the parent molecules, such as pyrazine. The vacuum chamber for the jet was evacuated by a 6 in. diffusion pump backed by an oil rotary pump. The operation of the pulsed jet nozzle was performed by our home-made nozzle driver. The nozzle driver also triggers a high voltage pulse power supply, Matsusada Precision Devices HJK-2P50-FU. The electric pulse of 2000 V, 50 /xs width was delayed from the electric pulse for opening the nozzle so that the glow discharge on the electrodes was synchronized with the expansion of molecules from the nozzle. The discharge creates metastable molecules, ions and free radicals. The triplet state molecules expanding into the vacuum chamber were effectively cooled through collision with a certain carrier gas. The pressure of the carrier gas was kept as low as possible because the high pressure of the gas considerably increases the emission from the metastable states of the carrier gas molecule comparing to the phosphorescence intensity of the molecule under study. The emission induced by the discharge was collimated with a large aperture quartz lens. The dispersed emission was detected with a Spex 1269 monochromator equipped with a photomultiplier tube of EMI-9789QB and the signal was processed with a gated integrator of Stanford Research Systems SRS250. Several kinds of carrier gas such as He, Ar and N 2 were used for the observation of the spectrum.
Y. Nibu et al./ Chemical Physics Letters 262 (1996) 615-620
Carriergas
617
[ NozzleDriver I
HighVoltage PowerSupply HJK-2P50-FU 2000V
SPEX-1269
\
SiC.View
Fq IR''r" I
Insulator ~ l ~ e~ t ct ~ t ~rt \°Nozzle I d E]~0Ite Front s View \ Electrodes'~~~~
+ ~ ~
Fig. 1. Apparatus used for the observation of the discharge phosphorescence spectrum in a supersonic free jet.
The bands due to the phosphorescence should be commonly observed for all the spectra with different kinds of carrier gas. Pyrazine and benzophenone from Wako were used without further purification.
3. Results and discussion
3.1. Dispersed phosphorescence spectrum of pyrazine Fig. 2 shows the dispersed emission spectra of pyrazine obtained by electric discharge in a super-
sonic free jet with He carrier gas under different observing conditions. The bands due to the metastable state of the carrier gas were picked up using the MIT wavelength table [21]. These bands were used for calibration of the bands of pyrazine phosphorescence. A strong band at 372.75 nm exactly coincides with the 0 - 0 band of the T~-S 0 transition of pyrazine observed in the gas phase in earlier works [5,6,11-14,15,17,18]. The spectrum emitted near the exit of the nozzle is shown in Fig. 2a. Extremely strong bands were observed around 387.5 nm. Through the observation of the high resolution spec-
618
Y. Nibu et aL / Chemical Physics Letters 262 (1996) 615-620
420
410
b) 0
420
410
400
_ 0 0 lOa 0 ~a 16a I
4 0 0 Llnm
390
9a 1o ©~0 /v~2
390
380
370
to~ 0
380
370
Fig. 2. Dispersed emission spectra of pyrazine observed by electric discharge excitation in a supersonic free jet with He. (a) Spectrum of the emission from the center of the nozzle exit and (b) spectrum of the emission far from the nozzle exit. The bands marked with * are due to the emission from metastable He.
trum, these bands were assigned to the rotational structure of the band origin of the transition from the B(22~ +, v' = 0) to X(2• +, u n = 0) states of the CN radical [20], which is one of the fragment species of pyrazine decomposed by the electric discharge. The same band structures were observed in the discharge spectra of CH3CN and pyrazine-d 4. These observations suggest that pyrazine is decomposed by electric discharge and the electronically excited CN radical is produced in a jet. The strong bands due to the CN radical and He overlap with the vibronic bands of the phosphorescence spectrum of pyrazine especially around 387.5 nm region as shown in Fig. 2a. Fig. 2b shows the spectrum observed the discharge emission from the region far from the center of the nozzle exit. The intensity of the bands due to the excited CN radical and He decreases drastically, but the intensity of the bands due to pyrazine phosphorescence does not decrease compared with those of CN and He. This observation can be explained as the difference in the lifetimes of the emitting species. The lifetime of the CN radical in the B state, 67.7 ns [22], is so short that the emission from the CN radical can only be observed near the center of the nozzle exit, where pyrazine is excited and the CN radical is created by the electric discharge. On the other hand, the lifetime of the lowest triplet state of
pyrazine is 4.45 ms [11] in a supersonic free jet and the triplet pyrazine can survive even in the region far from the nozzle exit. The observation was made by moving the collimator lens horizontally so that the emitting portion of the off-center jet was focused onto the slit of the monochromator. The 'space resolved spectrum technique' could be used for the effective selection of the phosphorescence of the molecule under study among emissions from many other species. The dispersed phosphorescence spectrum of pyrazine excited by the electric discharge in the jet gives higher resolution than those observed in earlier works made in a supersonic free jet. For example, a single band at 1830 cm-~ observed in the spectra of the previous works [10,12] splits into doubles, 1828 and 1840 c m - ~, in the present work. The former band is assigned to the 9a~6a] vibrations and the latter to the 10a 2 vibration. The bands due to the 9a t and 6a 2 vibrations are also clearly resolved, but were not resolved in the earlier works. The full-width at half-maximum (FWHM) of the 0 - 0 band of the discharge excited phosphorescence spectrum of pyrazine was about 0.15 nm and the vibronic structure was the same as those observed in the gas and cold matrix phases [23]. These observations indicate that the spectral structure of the pyrazine in both collision free and cold matrix phases has no intrinsic difference.
3.2. Phosphorescence spectrum of benzophenone The dispersed phosphorescence spectrum of benzophenone excited by electric discharge with He carrier gas is shown in Fig. 3. As the lifetime of the lowest triplet state of benzophenone is considered to be shorter than that of pyrazine judging from the lifetimes observed in a cold matrix, the phosphorescence intensity decreases sensitively by the change of the observing region by moving the collimator lens. The origin band of the discharge excited emission spectrum of benzophenone was observed at 412.92 nm, where the 0 - 0 band of the sensitized phosphorescence excitation spectrum of benzophenone was observed by Ohmori et al. [14] in a supersonic free jet. As can be seen in the figure, a low frequency progression spaced by 62 c m - i was observed. The
Y. N ibu et al. / Chemical Physics Letters 262 (1996) 615-620
s,i2 60
I
I
I
I
,I 420
1
I
2
i
o: 4 1 2 . 9 2 n m
60
6;
II
i
,
,
~./nm
c m "1
410
Fig. 3. Dispersed phosphorescence spectra of benzophenone in the 0 - 0 band region observed in a supersonic free jet. The bands marked with * are due to the emission from metastable He.
progression shows the mirror image relation with the progression observed in the phosphorescence excitation spectrum of benzophenone. These low frequency bands and the bands with 1700 c m - t spacings, which could be assigned to the C = O stretching vibration, form the main progression of the phosphorescence spectrum. These observations definitely give proof that the observed spectrum is the phosphorescence spectrum of benzophenone. The frequency of 62 c m - ' was assigned to the frequency of the totally symmetric torsional vibration of the two benzene rings in the ground state. The normal coordinate calculation shows that the frequency of the torsional vibration, whose mode is symmetric with respect to the C2(C=O) axis, is 57 cm -1 in the ground state [24]. The corresponding Raman band was observed as a weak shoulder band on the Rayleigh wing of benzophenone dissolved in benzene [24]. Seven quanta of the torsional vibration with constant spacing were observed in the low frequency progression. This observation indicates that the torsional vibration is strictly harmonic up to the higher vibrational levels. The same is also true for the vibration in the S I or T~ states [14,25-27]. The progression in the spectrum arises from the difference in the molecular structure between the ground and excited states along the symmetric torsional normal coordinate. Frederick et al. [25] and Kamei et al. [27] have made a Franck-Condon analysis of the vibronic structure of their spectra. They showed that the torsional angle of
619
each phenyl ring in the S I state decreases about 13° from that in the ground state due to the electric excitation. In their calculations, the frequency of 57 c m - l was used as the ground state frequency of the symmetric torsional vibration. The value of our experiment slightly changes the value of the calculated torsional displacement. The frequencies of the symmetric torsional vibrations of benzophenone in the S 0, S~ and T~ states are 62, 63 and 68 cm - f , respectively. The frequency of the torsional vibration could be affected by the strength of the 7r conjugation of the two phenyl rings through the carbonyl group. Thus the zr conjugation in the T~ state is stronger than the conjugation in the S O and S~ states. On the other hand, the FranckCondon maximum of the low frequency progression in the S j-S 0 transition is located at higher vibrational levels compared with that in the T~-S 0 transition. This fact suggests that the displacement of the torsional angle of the two phenyl rings from the equilibrium structure caused by the S ~-S 0 transition is greater than the displacement caused by the T~-S 0 transition. The excitation of the localized non-bonding electron into the delocalized 7r * orbital strengthens the conjugation of the two benzene rings. Therefore, the molecular structure in the excited states is closer to planar than in the ground state. The Franck-Condon analysis of the vibronic structure indicates that the degree of planarity in the electronic states is S~ > T, > S O. From the point of view of frequency, the order is T~ > S~ ~ S 0. The reason for the difference is now in question and the discrepancy of the results may be solved by a precise molecular orbital calculation of the excited states. The dispersed phosphorescence spectrum of benzophenone in a jet shows a considerably different structure from that observed in a cold solid matrix [28]. In the spectrum observed in 4,4'-dibromodiphenyl ether host crystal at 1.4 K, the intensity of the progression due to the symmetric torsional vibration having a frequency of 103 cm -~ was weak, although the intensity of the 0 - 0 band was strongest. This observation indicates that the molecular structure of benzophenone excited into the lowest triplet state in a cold matrix is different from that in a jet. Benzophenone has a flexible molecular structure for the torsion of the phenyl rings around the C(carbonyl)-C(phenyl) axis, and the structure is eas-
620
Y. Nihu et a L / Chemical Physics Letters 262 (1996) 615-620
ily affected by the perturbation from the surrounding molecules. The molecular structure of free benzophenone is considered to be n o n - p l a n a r in the S O state and more planar in the S 1 and T~ states [14,25-27]. Although the molecular structure in the jet is governed only by the intramolecular potential, the molecular structure in a cold matrix is governed by the balance of the inter- and intramolecular interactions. The spectral structure of the phosphorescence observed in a cold matrix suggests that the T ~ - S 0 excitation does not induce a considerable change in the molecular structure. This fact indicates that the molecular structure of b e n z o p h e n o n e is quite sensitive to the environment. In other words, the molecular structure of b e n z o p h e n o n e in the c o n d e n s e d phase is affected by the surrounding molecules. Therefore, the study of the solvation effect on the electronic spectrum of b e n z o p h e n o n e is expected to give information on the microscopic e n v i r o n m e n t a l effect on the molecular structure.
4. Conclusion The dispersed phosphorescence spectra of pyrazine and b e n z o p h e n o n e excited by electric discharge in a supersonic free jet show a highly resolved vibronic structure. The discharge emission spectroscopy would provide a useful method for studying the behavior of the lowest triplet molecules under collision free conditions. This technique is useful for the observation of low frequency vibronic structure such as internal methyl rotation. This method would be applicable to the observation of the emissions of metastable molecular species, such as free radicals or molecular ions. The application of this technique to metastable molecules will appear soon.
Acknowledgements This work was supported in part b y a G r a n t - i n - A i d for the E n c o u r a g e m e n t of Y o u n g Scientists by the Ministry of Education, Science and Culture.
References [1] See, for example, J.M. Hollas and D. Phillips, Jet spectroscopy and molecular dynamics (Blackie Academic & Professional, Glasgow, 1995), and references therein. [2] L.H. Spangler, Y. Matsumoto and D.W. Pratt, J. Phys. Chem. 87 (1983) 478 I. [3] L.H. Spangler and D.W. Pratt, J. Chem. Phys. 84 (1986) 4789. [4] L.H. Spangler, D.W. Pratt and F.W. Birss, J. Chem. Phys. 85 (1986) 3229. [5] A. Penner, Y. Oreg, E. Villa, E.C. Lira and A. Amirav, Chem. Phys. Lett. 150 (1988) 243. [6] J.L. Tomer, L.H. Spangler, K.W. Holtzclaw and D.W. Pratt, J. Chem. Phys. 88 (1988) 1528. [7] J.L. Tomer, L.H. Spangler and D. Pratt, J. Am. Chem. Soc. I10 (1988) 1615. [8] K.W. Holtzclaw, L.H. Spangler and D.W. Pratt, Chem. Phys. Lett. 161 (1989)347. [9] J.L. Tomer, S. Yamauchi and D.W. Pratt, Chem. Phys. Lett. 175 (1990) 3O. [10] C. Ottinger and A.F. Vilesov, Z. Physik. Chem. 188 (1995) Ill. [I 1] O. Sneh, D. Diinn-Kittenplon and O. Cheshnovsky, J. Chem. Phys. 91 (1989) 7331. [12] A. Penner and A. Amirav, J. Chem. Phys. 92 (1988) 5079. [13] E. Villa, A. Amirav, W. Chen and E.C. Lim, Chem. Phys. Lett. 147 (1988) 43. [14] N. Ohmori, T. Suzuki and M. lto, J. Phys. Chem. 92 (1988) 1086. [15] E. Villa, M. Terazima and E.C. Lim, Chem. Phys. Lett. 129(4) (1986) 336. [16] S. Sharpe and P. Johnson, J. Chem. Phys. 81 (1984) 4176. [17] H. lshii, M. Koyanagi and Y. Kanda, Bull. Chem. Soc. Jpn. 44 (1971) 1205. [18] I. lnoue and N. Ebara, J. Phys. Chem. 89 (1985) 4654. [19] N. Mikami, A. Hiraya, A. Fujiwara and M. Ito, Chem. Phys. Lett. 74 (1980) 531. [20] W.M. Jackson, J. Chem. Phys. 61 (1974) 4177. [21] F.M. Phelps III, M.I.T. Wavelength Tables, Vol. 2: Wavelengths by Element (The MIT Press). [22] C. Anastasi and D. Hancock. J. Chem. Soc. Faraday Trans. 2 84 (1988) 9. [23] F. Kokai and T. Azumi, J. Chem. Phys. 75 (1981) 1069. [24] J. Blazevic and L. Colombo. J. Raman Spectrosc. 11 (1981) 143. [25] J. Frederick E. Heller J. Ozment and D. Pratt, J. Chem. Phys. 88 (1988) 2169. [26] K.W. Holtzclaw and D.W. Pratt, J. Chem. Phys. 84 (1986) 4713. [27] S. Kamei, T. Sato, N. Mikami and M. ito, J. Phys. Chem. 90 (1986) 5615. [28] S. Yamauchi and D.W. Pratt, Mol. Phys. 37 (1979) 541.