J. Phys. Chem. 1986, 90, 6645-6648 the dimer, and either the monomer is chemically unstable or its spectrum w a not ~ detected under the conditions of the experiments performed. The dipolar interaction clearly carries more conformation-related information than the exchange interaction. The value of the isotropic exchange coupling corresponding to R = 3.36 A (analysis of spectrum S1) is Jo = 12.3 cm-'. From previous studies of the empirical cortelation between extremum values of magnetic exchange and the distance over which it occurs,16 we conclude that any value of R up to about 8.5 A is consistent with the observed (antiferromagnetic) value of Jo. To accurately correlate the experimental dipolar structural information with the "real" molecular structure would require a concurrent calculation of a set of atomic coordinates by an S C F molecular orbital or molecular (16) Coffman, R. E.; Buettner, G. R. J . Phys. Chem. 1979, 83, 2387. (17) Coffman, R. E.;Pezeshk, A. J. Magn. Reson. 1985,65, 62-81.
6645
mechanics energy minimization calculation of some reliability. The effects of delocalization of the unpaired electron must also be taken into account, in order to bring the dipolar R into agreement with the equilibrium structure resulting from such a calculation.
Acknowledgment. This research was supported by NIH Grant GM-24480. We gratefully acknowledge the assistance of Prof. Norman Baenziger in obtaining the unit cell structural information and the initial data for a structure. The numerical calculations were supported by the University of Iowa Weeg Computer Center. We also acknowledge the very important assistance to this project of the National Biomedical ESR Center, The Medical College of Wisconsin, Milwaukee, WI, for assistance with the L-band and S-band ESR spectra. Registry No. Cu(DIPS),(B) (B = pyridyl nitronyl nitroxide), 104716-49-6.
Observation of Autoiohization Spectrum of Kr 7df-5pf and 9sf-5pf Visible Transitions by Optogalvanic Spectroscopy Akihide Wada, Yukio Adachi, and Chiaki Hirose* Research Laboratory of Resources Utilization, Tokyo Institute of Technology, Midoriku, Yokohama, 227 Japan (Received: June 1 1 , 1986)
Several broad lines with widths (fwhm) from 5 to 30 cm-' have been observed on the optogalvanic spectrum of Kr in the frequency region from 16000 to 17 400 cm-I. From a double-resonanceexperiment, these lines are assigned to the transitions from 5p' levels to the autoionizing 7d' and 9s' levels which lie above the lowest 2P0j12 level of Kr+. Several additional weak autoionization lines which have been observed on the double-resonance signals on pumping the atom from the 5s levels to the 5p' levels are assigned. We estimate the number of Kr+ produced by the autoionization for a unit laser power and find that the ionization efficiency is about 2.4 times larger than the one caused by the direct photoionization alone in the region from 17 000 to 17 050 cm-I. A possible use of the two-step autoionization of Kr discharge for such fields as the preionization in excimer lasers is discussed.
1. Introduction Autoionization of atoms and molecules has been the subject of such basic science like the study of high-lying levels and photoionization and the diagnostics of astrophysical plasmas. The phenomenon has been investigated by vacuum ultraviolet absorption multiphoton absorption spe~troscopy,~-~ and electron impact spectroscopy.6 Recently, the observation by optogalvanic spectroscopy (OGS),which monitors the change in plasma impedance caused by the absorption of laser radiation by the species present within the plasma, has been reported for the one-photon transitions to the autoionizing levels from the excited states of Xe in radio-frequency (rf) discharge by using visible laser light.' The Rydberg series of krypton which converges to the first and of Kr+) has been resecond ionization limits (2P0 and 2P0112 ported in several article~,'-~+~~!ndsome of the levels are known (1) Yoshino, K.; Tanaka, Y. J . Opt. SOC.A m . 1979, 69, 159. (2) Hodgson, R.T.; Sorokin,P. 0.;Wygne, J. J. Phys. Rev. Lett. 1974, 32, 343. (3) Wygne, J. J.; Hermann, J. P. Opt. Lett. 1979, 4, 166. (4) Bradley, D. J.; Ewart, P.; Nickolas, V. J.; Shaw, J. R. D. J . Phys. B 1973, 6, 1594. ( 5 ) Nagvi, A. S.; Mirza, M. Y.; Semple, D. J.; Duley, W. W. Opt. Commun. 1981, 37, 356. ( 6 ) Cederquist, H.; Mannervik, S. J. Phys. B 1982, 15, L807. (7) Grandin, J.-P.; Husson, X . J. Phys. B 1981, 14, 433.
to autoionize to the 2P0312ion level. In this report, we describe autoionization lines that have been newly observed in the optogalvanic spectrum of a Kr hollow cathode discharge in the region from 16 700 to 17 250 cm-l. These lines get stronger upon simultaneously pumping the lower level from a metastable level by visible light, and their assignment has been given on the basis of optogalvanic double-resonance spectroscopy (OGDRS).I0 We also estimate the number of ions produced by autoionization and suggest that the autoionization may be adopted to the temporally and spatially controlled ionization of gases and be applied to the stable manipulation of preionization of a excimer laser by using a conventional dye laser. The preionization is ordinarily produced by electron beam bombardment or X-ray irradiation.
2. Experiment Section The Kr-filled discharge tube used in the present experiment was a homemade hollow cathode lamp of see-through type. The pressure of the Kr gas varied from 1 to 6 Torr. The cathode (8) Kaufman, V.; Humpherys, C. J. J . Opt. SOC.A m . 1969, 59, 1614. (9)Delsert, C. D.; Keller, J.-C.; Thomas, C. J. Phys. B 1981, 14, 3355. Aymar, M.; Robaux, 0.;Thomas, C. J . Phys. B 1981, 14, 4255. Delsart, C.; Keller, J.-C.; Thomas, 0. J. Phys. B 1981, Z4, 4241. Dunning, F. B.;Stebbings, R. F. Phys. Rev. A 1974, 9, 2378. (10)Miyazaki, K.; Scheingraver, H.; Vidal, C. R. Phys. Rev. A 1983, 24, 2229.
0022-3654/86/2090-6645$01.50/00 1986 American Chemical Society
Wada et al.
6646 The Journal of Physical Chemistry, Vol. 90, No. 25, 1986 M
P
12
RECORDER
L..
E.
-?S'
5d'
11 -
%
6D'
-
7s
P
5d
El
g 1c 4
6s
k Q
B.S.
4
\
'
4d
5P'
W Z W
.P.
DYE LASER
A;
LASER
9
Figure 1. Schematic of experimental setup of the optogalvanic doubleresonance spectroscopy (OGDRS). B.S., beam splitter; E., Fabry-Perot etalon; F.P., Fabry-Perot interferrometer; L., lens, M., mirror, P.D., photodetector.
cylinder which was made of brass was 4.5 mm i.d. and was 20 mm in length. The discharge current ranged from 2 to 15 mA, and the interelectrode voltage was about 230-300 V. The laser source used mainly was the linear dye laser (Coherent Radiation 590, line width of about 30 GHz) pumped by an Ar+ laser (NEC GLG-3302). A ring dye laser (Spectra Physics 380, line width of about 20 MHz) pumped by an Ar+ laser (Spectra Physics 164) was used as the pump laser in the OGDRS. The output power of the dye lasers was about 50 mW, and the beam diameter was less than 1 mm. The experimental setup of OGDRS is shown in Figure 1. The frequency and intensity of the pump laser, which were set resonant with the transitions from the metastable %(3/2)'1 level to one of the 5p' sublevels, namely 5 ~ ' ( 3 / 2 ) ~5p'(1/2)', , and 5p'(3/2),, were watched by the optogalvanic signal obtained from another Kr-filled discharge tube. A partial Grotian diagram which shows the relevant levels of Kr is shown in Figure 2. The two laser beams, frequency-fixed pump beam and frequency-scanned probe beam, were separately modulated a t 5 10 and 7 10 Hz by a mechanical chopper and crossed inside the cylindrical hollow-cathode electrode. A Fabry-Perot etalon (free spectral range of about 300 GHz) was used as the frequency marker of the probe beam. The OG signal was picked up through a coupling capacitor of 0.1 p F from the anode which was connected to the voltage-controlleddc source via a ballast resistor of 50 kQ and was detected by phase-sensitive detection by using a lock-in amplifier (Brookdeal 9503) at the sum frequency of the two modulation frequencies of 1220 Hz. 3. Results The one-photon optogalvanicspectrum of Kr as seen in Figure 3, in which only the region of the appearance of autoionization lines is shown, reveals several broad lines which are marked by * among much sharper bound-bound lines of Kr. The fwhm of these lines ranges from 8 to 30 cm-', and the lines appeared only when the laser beam was passed through the cathode dark space which extends to about 1.2 mm deep from the cathode surface. Their height and the width were independent of the gas pressure (from 1 to 6 Torr) and the discharge current (from 2 to 15 mA). They were not resolved by the experiment using the ring dye laser that has the line width at about 20 MHz. The possibility that the broad lines originate from a molecular species is discarded since the separations among the observed lines are quite irregular and the lines that can be attributed to the vibrational satellites are absent in the frequency region from 16 700 to 17 250 cm-'. From these facts and the results of OGDRS which are described below, we concluded that the broadened lines are atomic lines and the broadening is due to the autoionization of the upper levels.
-5s'
1/2
1
-5s'11/210
a
-5 s ( 3 / 2 ) 2 -5s ( 3 / 2 ) 1
Figure 2. Partial Grotian diagram of krypton which shows the relevant levels that converge to the ionization limits of and 2P03/2. The electron configuration of the inner core (KLM 4s24p5)is omitted, and only that of the optical electron is indicated. The primed configuration converges to the 2P,/2term of Kr+. The full notation in the $-coupling scheme, which is appropriate for Kr, is given only for the lowest excited levels, 5s and 5s' configurations. The shaded blocks indicate the groups of the sublevels.
Figure 3. One-photon optogalvanic spectrum of Kr observed by irradiating the cathode dark space. Only the region where the autoionization lines are observed is shown. The lines marked by * are absent when the laser beam passes through the negative glow region.
On the intermodulated OGDR spectrum, an example of which is shown in Figure 4, nine broad lines were observed only when the frequency of the pump laser was set resonant with either 5~'(3/2)~-5~(3/2)'~, 5p'( 1/2)1-5s(3/2)01, or 5~'(3/2)~-5~(3/2)'~. The OGDR intensity showed strong dependence on the choice of the pumped level as seen in Figure 4 which is the OGDR spectrum in the region of 17 150-17 300 cm-I obtained for the three pumping schemes. Sharp lines marked by correspond to the bound-bound transitions of Kr, and broad lines marked by * are the autoionization lines. The pumping of the 5 ~ ' ( 3 / 2 )level ~ gives rise to the lines at 17 190,17216, and 17237 cm-'. The broad lines at 17204 cm-I become visible when the 5p'( 1/2)' level is pumped. Pumping of the 5p'(3/2), level gives only the 17 237-cm-I line. The results of the present OGDR experiment are summarized in Table I, where the appearance of the signal for each pumping scheme is indicated by 0.Some of the lines which are marked by * in Table
+
The Journal of Physical Chemistry, Vol. 90, No. 25, 1986 6647
Observation of Autoionization Spectrum of Kr TABLE I: Result of OGDRS“
pumped level 5P’(3/2)z
16783
16 888
16933
0
0 0
0 0
-
1/2) I 5P’(3 / 2) I
-
spectral line, cm-I 17 074 17 134 17 190
-
0
-
-
17 204
17 216
17237
0
0 0
0 0 0
-
0 0
-
-
0
~~
-
“The appearance and absence of the intermodulated OGDR signals by the pumped levels (shown on the first column) are indicated by o and -, respectively. TABLE 11: Assignment of Autoionization Spectrum of Kr Observed in Visible Region
position of spectral lines,” cm-l
TABLE III: Number Density of Kr+ Produced by Two-step Autoionization and Photoionization of Kr*
assignment position of spectral line, cm-I 17 190
16933 *17074 *17 134 17 190
7d‘(3/2)’,-5~’(3/2)2
*17204
9s’(1/2)0~-5p’(1/2),
‘17216 17 237
9S’(
7d’( 5/2)02-5p’( 3/2) I 9~’(1/2)’,-5p’(3/2), 1/2)O&’( 1/2)1 7d‘( 3/2)’2-5~‘( 3/2) I
“The lines which are so weak and become observable only on the double resonance are indicated by *.
16933 16 888 16783 photoionization‘
density,” cm-3 w-l 4 x 108 3 x 109 7 x 108 6 X lo7 2 x 109
W+I/[Krl 7x 4x 1x 7x 5 x
10-9
10-8 10-8 10-10
10-8
“The values that are extrapolated for the irradiation by the probe laser with the total power of 1 W contained in a rectangular spectral profile of 1-cm-’ width are given. bSince the ionization under consideration is due to the processes starting from the metastable levels by using the light whose power density is 1 W/cm-I, the cited values, which are the ratio to the total number of atoms in the ground state, should be highly dependent on the conditions of the discharge that prepares the metastable Kr*. cThe estimation is performed from the background signal over the region from 17 000 to 17 050 cm-I. discharge conditions has enabled us to derive a relation between the discharge voltage and the ion density in the cathode dark space. The results for the discharge under a pressure of 2 Torr give the relation
n+ = 3.5
17300
17250
17100
171’50cfil
Figure 4. Optogalvanic double-resonance spectrum of Kr. The lines
marked by + correspond to the bound-bound transitions of Kr, and the autoionization lines are indicated by *. (a) 5p’(3/2)2 level is pumped. (b) 5p’(1/2)1 level is pumped. (c) 5p’(3/2)1 level is pumped. I1 are too weak to be observed without pumping the lower levels. When the OGDRS observation is combined with the available term values determined by Kaufman and Hamphreys* and Yoshino and Tanaka’ and with the selection rule, the lines at 17 190 and 17216 cm-’ are assigned to the 9 ~ ’ ( 1 / 2 ) ~ , - 5 ~ ’ ( 3 / 2 ) ~ transition and the 9s’( 1/2)01-5p’( 1/2)’ transition, respectively. The line a t 17 204 cm-’ is assigned to the 9~’(1/2)~,-5p’(1/2), transition. It is natural now to attribute the remaining lines to the transitions from 5p’ to 7d’ configurations. Among the sublevels of the 7d’ configuration, only the 7d’(3/2)Oi level has been reported and lies at 17073 cm-I above the 5p’(3/2), level. The results of the present experiment and the frequency separations among the observed lines were combined with the consideration of the transition probabilities calculated on the basis of the jl-coupling scheme to give the assignment as shown in Table 11. 4. Discussion
The electric field strength in the cathode dark space of the Kr hollow-cathode discharge has been obtained from the analysis of the Stark effect,” and extensive investigationi2 under various (1 1) Kawakita, K.; Nakajima, T.; Adachi, Y.; Maeda, S.; Hirose, C. Opt. Commun. 1983, 48, 121.
1O*V- 7.6 X loio (1) where n+ is the averaged positive ion density ( ~ m - ~over ) the cathode dark space and Vis the voltage applied between cathode and anode. The standard deviation for fitting the observed data . 1 enables us to estimate the by eq 1 is 1.1 X lo9 ~ m - ~Equation laser-induced change of ion density from the magnitude of OG signal, which is given in millivolts. We assumed Lorentzian and rectangular line shapes for the autoionization spectrum and the laser radiation, respectively, and the estimation is made by linear extrapolation to the integrated laser power of 1 W/cm-l in the rectangular profile of the laser used in the second step of excitation. Deconvolution of the observed signal and the integrated area intensity of the deconvoluted lines gave the result as listed in Table 111. In the third column of Table 111, the ratio of the number of produced ions to the total number of Kr atoms at a pressure of 2 Torr is also shown. The density of ions produced by direct photoionization of the atoms in the excited states is estimated from the background signal which persists over the region from 17 000 to 17 050 cm-’, and the values are shown in the last row of Table 111. It is mentioned here that the values listed in Table I11 are derived on the assumption that the ions are uniformly distributed over the cathode dark space. The ion density inside the laser beam, which operates as a sort of ion source, should be much higher. From the fwhm of each autoionization line, the lifetimes of the autoionizing levels of the 7d’ and 9s’ configurations are estimated to lie between 1 and 17 ps. The short lifetime indicates that the atoms excited to these levels undergo the autoionization with quite high probability. We thus find that Kr a t o m can be excited to autoionizing levels by two-step excitation from the metastable levels by visible light. Considering that the production of the metastable atoms can be easily achieved by a conventional discharge and following excitation to the autoionizing levels is effectively carried out by using a commercially available light source at the frequencies of easiest control, the ionization of rare gases by the combination of glow X
(12),Fujimaki, S.; Adachi, Y.; Hirose, C., unpublished results. Ion density was derived from the electric field strength in the cathode dark space that was determined from the Stark effect.
6648
J. Phys. Chem. 1986, 90, 6648-6654
discharge and visible light will be a practical and inexpensive method for the preionization in such devices as excimer lasers. Simple configurations that produce the ions with comparatively good efficiency will also be useful in the field of ion etching and others. The exact determination of the term values of the autoionizing levels requires the computer fitting of several overlapping Fano pr0fi1es.l~ Detailed analysis is in progress at the moment, and
the result will be published in a forthcoming paper.
Acknowledgment. We are grateful to Prof. Shiro Maeda and Dr. Shunsuke Kobinata for their helpful discussions. The kind offer of Komatsu Ltd. to use the ring dye laser, which was crucial in carrying out OGDRS, is also gratefully acknowledged. (13) Fano, U. Phys. Rev. 1961, 124, 1866.
Ultraviolet Resonance Raman Spectroscopy of Flavln Mononucleotide and Flavin Adenine Dinucleotlde Robert A. Copeland and Thomas G. Spiro* Department of Chemistry, Princeton University, Princeton, New Jersey 08544 (Received: June 13, 1986)
Raman spectra are reported for aqueous flavin mononucleotide (FMN) excited with a Nd:YAG pulsed laser and H2 Raman shifter at 355, 266, 240, 218, and 200 nm. They are compared with the Raman spectrum obtained by 488-nm CW Art laser excitation, with KI as a fluorescence quencher. No quenching is needed for the UVRR spectra, since the fluorescence is shifted sufficiently to the red that the Raman bands are not obscured. These wavelengths span the four major absorption bands associated with the ?r system of the flavin chromophore, at 450, 370, 266, and 220 nm. Significant differences are observed in the relative intensities of the RR bands associated with the differing excited-state distortions. New flavin RR bands are seen with UV excitation which have not previously been detected with visible excitation RR studies. The spectrum excited at 240 nm, in the valley between the 266- and 220-nm absorption bands, is surprisingly strong and is not a superposition of the 218- and 266-nm-excited RR spectra. This spectrum is suggested to arise from resonance with a weak H-T* transition at -240 nm which mixes vibronically with the nearby fully allowed transitions. The 218-nm-excited spectrum is particularly well resolved and has been used to reexamine the question of changes in frequency upon substitution of CI for CH3 at the C8position and of D for H at the N3 position. The effects of N3H/D exchange on the bands in the 1100-1300-~m-~region are more complex than previously thought, and there is clearly substantial alteration in the composition of several normal modes. The low-frequency region of the 266-nm-excited spectrum is particularly rich, and several new bands below 1000 cm-' have been detected. The more complete band frequency list available from these spectra reveals discrepancies in previous normal-coordinate analyses of flavin. Raman spectra are also reported for flavin adenine dinucleotide (FAD) excited at 266,240,218, and 200 nm, which show varying contributions from the flavin and adenine chromophores. Raman hypochromism is found for bands of both chromophores, consistent with a stacking interaction between them.
Introduction anti-Stokes Raman spectroscopy) techniq~e,'.~ natural quenching of the fluorescence by the protein host,4 addition of external Flavins play a key role as redox cofactors in biology.' Figure quenching agents, e.g. KI,9 and quenching by adsorption onto 1 is a structural diagram for the common flavin cofactors, all based metal particles, particularly Ag, which also produces strong enon the isoalloxazine chromophore, which is capable of facile onehancement of the Raman signal via the SERS (surface-enhanced and two-electron reductions. The extended s electronic system Raman scattering) effect.I0 All of these methods, however, have of this chromophore gives rise to strong r-r* electronic transitions significant limitations. Most flavoproteins do not give complete in the visible and ultraviolet regions.2 Laser excitation in these quenching of the flavin fluorescence. External quenchers are transitions is expected to produce resonance enhancement of ineffective for flavins bound inside proteins (and may also denature Raman transitions associated with isoalloxazine vibrational modes. the proteins). Adsorption on silver particles is much more effective, The resonance Raman technique is capable of providing detailed but the consequences of adsorption for the protein structure must structural information about the interaction of flavin cofactors be evaluated in each case. CARS spectroscopy provides a general with their protein environment and with substrates or inhibitors. solution to the fluorescence problem, but the technique is techRaman spectra excited in the longest wavelength absorption, nically difficult, and the spectra contain distorted Raman bands -450 nm, which is accessible to commonly available CW lasers, because of interference with background electronic processes which are subject to interference by intense flavin fluorescence. Various are pronounced under resonance conditions."J2 These distorted methods of reducing this interference have been found, and the lines can be analyzed to give proper vibrational frequencies,"J2 visible-excitation R R spectrum of flavin has been well catalogued3 and analyzed via isotope substitution4 and normal-mode a n a l y s i ~ . ~ ~ ~ These methods include the application of the CARS (coherent (7) (a) Dutta, P. K.; Nestor, J. R.; Spiro, T. G. Proc. Nutl. Acad. Sci. ( I ) Walsh, C. Enzymatic Reaction Mechanisms; W. H. Freeman: San
Francisco, CA, 1979; pp 358-448. (2). Eaton, W. A.; Hofrichter, J.; Makinen, M. W.; Andersen, R. D.; Ludwig, M. L. Biochemistry 1975, 14, 2146. (3) McFarland, J. T. In Biological Applications of Raman Spectroscopy; Spiro, T. G., Ed.; Wiley: New York, in press. (4) Kitagawa, T.; Nishina, Y.; Kyogoku, Y.; Yamano, T.; Onishi, N.; Takai-Suzuki, A.; Yagi, K. Biochemistry 1979, 18, 1804. ( 5 ) Bowman, W. D.; Spiro, T. G. Biochemistry 1981, 20, 3313. (6) Abe, M.; Kyogoku, Y. Spectrochim. Acta, Part A , in press.
0022-3654186 12090-6648%01SO10 , ,
I
U.S.A.1977, 74,4146. (b) Dutta, P. K.; Nestor, J. R.; Spiro, T. G. Biochem. Biophys. Res. Commun. 1978, 83, 209. (8) Muller, F.; Vertoot, J.; Lee, J.; Horowitz, M.; Carreira, L. A. J . Raman Spectrosc. 1983, 14, 106. (9) Benecky, M. J.; Li, T. Y.; Schmidt, J.; Frerman, F.; Watters, K. L.; McFarland, J. T. Biochemistry 1979, 18, 3471. (10) (a) Copeland, R. A.; Fodor, S. P. A.; Spiro, T. G. J . Am. Chem. SOC. 1984, 106, 3872. (b) Fodor, S. P. A.; Copeland, R. A,; Spiro, T. G., manu-
script in preparation. ( 1 1 ) Dutta, P. K.; Spiro, T. G. J. Chem. Phys. 1978, 69, 3119. ( I 2) Carreira, L. A.; Antcliff, R. A. In Advances in Laser Spectroscopy; Garetz, B. A., Ed.; Heyden: London, 1982; p 21.
0 1986 American Chemical Society
