J . Phys. Chem. 1992, 96,6124-6126
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Observation of Stark Quantum Beats for Slngle Rotational Level Excltation of SO2 Nobubiro Oh@* Iwao Yamazaki, Department of Chemical Engineering, Faculty of Engineering, Hokkaido University, Sapporo 060, Japan
Takeshi Takemura, Division of Chemistry, Research Institute of Applied Electricity, Hokkaido University, Sapporo 060, Japan
and Takehiko Tanaka Department of Chemistry, Faculty of Science, Kyusyu University, Fukuoka 812, Japan (Received: April 20, 1992; In Final Form: June 1, 1992)
Stark quantum beats are observed in the fluorescence decay following excitation at the 'RO(O)rotational line belonging to the Clements' "E" letter band of the AIAz-XIAItransition of SOzin a supersonicjet. The beat frequencies depend quadratically on the electric field, and the electric dipole moment in the excited state is evaluated to be 2.66 D.
Introduction
Quantum beat spectroscopy is a Doppler-free technique, which makes possible a precise measurement of small energy separation between pairs of coherently excited levels.' This technique combined with Stark splittings, Le., Stark quantum beat spectroscopy, is a useful way to determine excited-state dipole moments, but it has been successfully applied so far only to a limited number of diatomic m0lecules,2-~NO2: propynal,' formaldehyde,* and pyrimidine? Stark quantum beats were observed also for CSZ,Io but the dipole moment of the excited state of this molecule has not been evaluated. In this letter, we report the observation of Stark quantum beats in the fluorescence of SOz excited at a single rotational line belonging to the "E" band (Clements' notation) of the A1A2-X'A1 transition. The electric dipole moment in the excited state is evaluated, and the value is compared with that in the ground electronic state. Experimentd Section
The experimental apparatus and procedures are essentially the same as reported in a previous paper.I1 The second harmonics of a dye laser (Lambda Physik FL2002E) pumped by a XeCl excimer laser (Lambda Physik EMG 103 MSC) having a spectral width of less than 0.1 cm-l and a pulse duration of 10 ns was used for excitation. A pulsed molecular beam was formed by expanding SOz in He at a concentration of less than 5% with a total pressure of 600 Torr through a nozzle of 0.2-mm diameter. The sample was excited at 30 mm downstream from the nozzle exit. A traditional orthogonal geometry was employed for the measurements. A molecular beam axis and the excitation light, which crosses at right angles, define the X and Y axes, respectively. Linearly as well as circularly polarized laser light is used for excitation. A Babinet-Soleil compensator (Oyo Kaden) converts the light from the laser, which is originally linearly polarized parallel to the X axis, into circularly polarized light or rotates the plane of polarization by 90°. Undispersed fluorescence is collected through a spherical lens with a diameter of 50 mm, which is placed at a distance of 50 mm from the excitation position toward the Z direction. A semicircular area of the lens is masked, so that fluorescence emitted to one side of the YZ plane (e.g., X > 0) is blocked. As occasion demands, the mask is removed. It should be noted that the lens can collect fluorescence emitted to directions substantially slanted from the Z axis. The fluorescence is monitored through a polarization analyzer oriented to transmit emission with electric vector parallel to the X or Y axis and then through a scrambler. A pair of parallel Stark plates, one of which is transparent to the fluorescence, generates an electric field along the Z axis at
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0022-3654/92/2096-6124303.00/0
the excitation position. Hereafter, the electric field is denoted by E,. Fluorescence decays were measured with a digital memory (Iwatsu DM901). Fourier transformation of the decay curves was carried out by using the first 512 data points, each of which has a gate time of 10 ns.
Results and Discussion Fluorescence decays reported in this letter were measured by fitting the excitation wavenumber to the top of the rotational line at 32 872.5 1 an-',whose line number is l2.IZl3 This line has been assigned as the '&(O) line of the E band by Tsuchiya and coworkers.1213 It is noted that the fluorescence excitation spectrum around 32 870 cm-I observed in the present study is essentially the same as the ones reported by Tsuchiya and c ~ - w o r k e r s ~ ~ J ~ and by Suzuki et al.I4 Figure 1 shows fluorescence decays in the presence of E, of 1.OO kV/cm (a and b) and in the absence of E, (c), obtained by employing a circularly polarized excitation light. Hereafter, this excitation is denoted by (Tu)-excitation. The polarization direction of the observed emission is along the X axis in Figure la,c and along the Y axis in Figure lb. All these decays were obtained by monitoring the fluorescence collected from only one side of the YZ plane. For the '&(O) line excitation, Stark quantum beats were observed only when (Ta)-excitation was employed and only when fluorescence polarized along the X axis was monitored. However, Stark quantum beats were not observed in the fluorescence collected from both sides of the YZ plane, which was obtained by removing the semicircular mask of the lens, even when fluorescence polarized along the X axis was monitored with (Tu)-excitation. These results indicate that fluorescence emitted to one side of the YZplane and that emitted to the other side carry beat signals with opposite phases. Fluorescence decays were also observed with a linearly polarized excitation light whose polarization direction is along the X or Z axis. These excitations are denoted by u- and *-excitations, respectively. With these excitations, however, Stark quantum beats were not observed for 'RO(O),in contrast with (Tu)-excitation. Figure 2 shows the Fourier transforms of the beating decays observed with (Tu)-excitation for different strengths of E,. Only one beat was confirmed at each strength of E,. The plot of the beat frequency as a function of the square of E, shows a straight line with a slope of 12.26 MHz k V 2 cmz, as is shown in Figure 3.
In SOz,which is a planar C, near prolate top, only the dipole moment along the b inertial axis, which is denoted by Pb, has a nonzero value. Then, the Stark energy shift given by the second-order perturbation is expressed as follows: W(JK&~J) = (a + 8M?)rbzEzz (1) 0 1992 American Chemical Society
The Journal of Physical Chemistry, Vol. 96, No. 15, 1992 6125
Letters
c4
30-
N
/
I
I
/ O
v
u"
20-
12.26 MHz kf2cmz
aJ
O/O O/
U J
2
LL
10-
/O' /O /O O/O
0
I
E2=1.00 kV/cm
Ez= 0 kVkm
0
2
I
Time (/us) Figure 1. Fluorescence decay profiles of SO2with excitation at the '&(O) line of the E band in the presence of 1.00 kV/cm (a and b) and at zero field (c). Fluorescence polarized along the X axis was observed in a and c, and fluorescence polarized along the Y axis was observed in b. In every case, (Tu)-excitation was employed, and only the fluorescence emitted to one side of the YZ plane was monitored. E2=1.40 kVlcm
t
I
E2=1.20 kVkm
aJ
D J
I
tI E2=1.00 kVlcm
E2.0.60
0
IO
20
kVlcm
30
40
Frequency ( MHz ) Figure 2. Fourier transforms of the beating fluorescence decay following excitation at '&(O) of the E band with (xu)-excitation at 0.60,1.00, 1.20, and 1.40 kV/cm (from bottom to top). Beat positions are indicated by an arrow.
for the magnetic sublevel of Mj of JKae.Is The rotational level with J' = 1, K,' = 1, K,' = 0 (1 which is prepared by excitation at '&(O), can couple only with the levels
'
O / O
C
/d
I
J . Phys. Chem. 1992,96, 6126-6128
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in the excited state to be 2.66 D, since fl is calculated to be 1.7362 Thus, the dipole moment in the excited state, Le., 2.66 D, is much MHz k V 2 cm2 D-2, as mentioned above. larger than in the ground state. A large increase of the dipole The present experimental setup for the measurements of the moment accompanying excitation from the ground state to the Stark quantum beat was adequate enough to determine the dipole electronically excited state seems to be correlated to a larger S - 0 moment of the excited state. However, it should be noted that bond length and a smaller OS0 angle in the excited state than the beat signals are not maximized. Experimental arrangements in the ground electronic state. In fact, the bond length and the which maximize the modulation depths of the quantum beats are angle in the excited state were shown to be increased by about reviewed by Hack et al.” 0.1 A and decreased by about 20°, respectively, at this excited The lpbl value of 2.66 D was obtained by assuming that the electronic state.I 8 pertinent rotational levels belong to the same vibrational state References and Notes which is not perturbed by background levels. In such a case, (1) See for a review: Haroche, S.In High-Resoluiion Laser Spectroscopy; energies of individual rotational levels are expected to be calculated Shimoda, K., Ed.; Topics in Applied Physics Vol. 13; Springer-Verlag: Berlin, with proper rotational constants. For the E state, the rotational 1976; p 253. constants were given by Hamada and M e r e P as A’ = 1.178, B‘ (2) Brieger, M.; Hese, A.; Renn, A.; Sodiek, A. Chem. Phys. Leu. 1980, = 0.350, and C’ = 0.278 cm-I. By using these constants, the 76, 465. (3) Brieger, M.; Hese, A.; Renn, A.; Sodiek, A. Chem. Phys. Lett. 1981, zero-field energies of ll0, lO1,and 221are calculated to be 1.528, 78, 153. 0.628, and 5.340 cm-I, respectively: they are given by A ’ + B’, (4) Schweda, H. S.; Renn, A.; Hese, A. Chem. Phys. 1983, 75, 1. B’+ C’,and 4A’+ B’+ C’, respectively. By using these zero-field (5) Busener, H.; Heinrich, F.; Hese, A. Chem. Phys. 1987, 112, 139. energies, W (1 - W (lol)and W (1 - W(2*J are calculated (6) Brucat, P. J.; Zare, R. N. Mol. Phys. 1985, 55, 277. (7) Schmidt, P.; Bitto, H.; Huber, J. R. J . Chem. Phys. 1988, 88, 696. to be 0.90 and -3.81 cm-I, respectively. These values are close (8) Vaccaro, P. H.; Zabludoff, A.; Carrera-Patino, M.E.; Kinsey, J. L.; to, but not consistent with, the above-mentioned values, 1.30 and Field, R. W. J. Chem. Phys. 1989, 90, 4150. -3.86 cm-’, respectively, which were obtained from the data of (9) Ohta, N.; Takemura, T.; Tanaka, T. To be submitted for publication. fluorescence excitation spectrum in a jet. Accordingly, we may (10) Warnaar, D. L.; Silvers, S. J. Chem. Phys. Lett. 1991, 184, 383. (1 1) Ohta, N.; Takemura, T. Chem. Phys. Lett. 1990, 169, 61 1. have to expect a little error for the present analysis, since the (12) Watanabe, H.; Tsuchiya, S.; Koda, S.Faraday Discuss. Chem. Soc. rotational levels under consideration may be a little perturbed by 1983, 75, 365. background levels. In fact, interaction between zero-order vi(13) Watanabe, H.; Tsuchiya, S.; Koda, S. J. Mol. Spectrosc. 1985, 110, brational states of the IBl(nn*) and ‘Al(mr*) states, Renner136. (14) Suzuki, T.; Ebata, T.; Ito, M.; Mikami, N. Chem. Phys. Lett. 1985, Teller coupling of the IB, state with high vibrational levels of the 116, 268. electronic ground state, singlet-triplet interaction, and Coriolis (15) Towns, C. H.; Schawlow, A. L . Microwave Spectroscopy; McGrawcouplings were suggested to be operating in the excited ~ t a t e . ~ ~ J * J Hill: ~ New York, 1955. These problems seem to be elucidated by further studies of dipole (16) Lovas, F. J. J . Phys. Chem. Ref. Data 1985, 14, 395. (17) Hack, E.; Bitto, H.; Huber, J. R. 2.Phys. D 1991, 18, 33. moment determinations for other levels of the excited state under (18) Hamada, Y.; Merer, A. J. Can. J . Phys. 1974, 52, 1443; 1975, 53, consideration, which are now in progress. 2555. The dipole moment in the ground electronic state has been (19) Kullmer, R.;Demtriider, W. J . Chem. Phys. 1984, 81, 2919. determined to be 1.633 D, based on microwave spectroscopy.20 (20) Patel, D.; Margolese, D.; Dyke, T. R. J . Chem. Phys. 1979,70,2740.
I R Spectra of CO and NO Adsorbed on Cgg M. Fastow,*?+Y. Kozirovski? M. Folman? and J. Heidberg1 Chemistry Department, Technion-ZIT, Haifa 32000, Israel, and Institute fur Physikalische Chemie und Elektrochemie der Universitat Hannover, Hannover, Federal Republic of Germany (Received: April 16, 1992)
The adsorption of CO and NO on c60 is studied using the technique of IR spectroscopy. The IR spectra of CO show two absorption bands, which indicate the presence of two adsorption sites on the C,, surface. The IR spectra of NO show, in addition, a multiplicity in the two absorption bands, which indicates that NO is adsorbed in its dimer form on two sites. Large spectral shifts are observed on adsorption of both CO and NO on C60, demonstrating that the interaction of these gases with c60 is relatively strong.
Introduction Despite the large amount of research devoted to C60since its discovery and preparation in macroquantities, to our knowledge no adsorption studies of this material have been reported. In this study, the IR spectra, of adsorbed CO and NO, at low temperatures, were recorded, interpreted, and compared to the respective IR spectra in the gas phase and to the absorption spectra of CO and NO on graphite and diamond films. Application of IR spectroscopy posed some difficultics in the study of adsorbates on graphite films, due to the low transparency of this material. This difficulty was not encountered in the case Technion-IIT. ‘Universitat Hannover.
0022-365419212096-6126$03.00/0
of Cs0and diamond films, where the transparency was much higher.
Experimental System Thin films of Csodeposited on KBr windows were prepared from purified C,, supplied by M.E.R. Co., T m n , AZ. The adsorbent films were evaporated, in a 6-Torr helium atmosphere, from a small tantalum furnace, which was heated to 673 K. The C,, deposited on a KBr window was mounted in a low temperature adsorption cell, which was connected to a vacuum system and to the supply of the gas adsorbates. The cell was mounted inside a dewar furnished with a pair of windows transparent to IR radiation. During adsorption, the cell was cooled with liquid nitrogen. The spectra were recorded by means of P.E. 580B and 0 1992 American Chemical Society
