J . Phys. Chem. 1987, 91, 2589-2592
2589
Multiphoton Ionization of Jet-Cooled Iodine John C.Miller Chemical Physics Section, Health and Safety Research Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831 (Received: June 17, 1986)
The spectroscopy of I,, rotationally and vibrationally cooled in a supersonic expansion, was probed by multiphoton ionization mass spectroscopy. A new band system, in the vicinity of the Dalby bands, has been characterized and the upper level tentatively assigned to the 0,(211,)nu state. Fragmentary spectra of the B 3110+u valence state and other unidentified gerade Rydberg states have also been observed. No evidence for I,-rare gas clusters was obtained, presumably because of fast predissociation channels.
Introduction Molecular iodine was one of the first molecules to be studied by the techniques of multiphoton ionization (MPI) spectroscopy.' Its low ionization potential, reasonable vapor pressure, and homonuclear nature provided the elements needed to demonstrate the utility of the MPI technique. A new, two-photon-allowed Rydberg state was discovered and Characterized,' thus dramatically illustrating the changed selection rules for a two-photon process compared to conventional one-photon spectra. The sensitivity and experimental ease of the technique were also clearly apparent. Later studies, characterized another two-photon transition at lower energy assigned as the spin-orbit-split companion to the Dalby bands. As mass3 and photoelectron spectroscopy4 were coupled to molecular MPI, I2 was again a prototype system for study. The iodine MPI was found to result primarily in I+ ions with some 12+detected when exciting lower energy vibration^.^ Photoelectron spectra4 indicated that the I,+ ion rapidly absorbs a fourth photon into a dissociative state, thus producing 1' ions. In the same time frame, the development of supersonic expansion techniques for molecular spectroscopy illustrated the advantages of rotational and vibrational cooling. In addition, the ready formation of clusters in such an expansion initiated the rapidly growing field of the spectroscopy of van der Waals clusters. Again, cold and 12-rare gas molecules' have been extensively studied, chiefly by laser-induced fluorescence techniques. In the present work, supersonically cooled iodine has been reinvestigated by MPI techniques. TKe original intent was to search for Rydberg states of the 12-rare gas molecules, such as those recently characterized for nitric oxide-rare gas cluster^.^*^^ No evidence for these species was obtained but several new states of 1, have been characterized and possible assignments are discussed. Definitive assignment of these new states, however, awaits further experimental and theoretical work. Experimental Section The apparatus used here has been described in detail in recent paper^.^,^ Briefly, the beam of an excimer pumped dye laser (Lambda Physik) was focused with a 75-mm lens into the ex( I ) Petty, G.; Tai, G.; Dalby, F. W. Phys. Rev. Lett. 1975, 34, 1207. Dalby, F. W.; Petty-Sil, G.; Pryce, M. H. L.; Tai, C. Can. J . Phys. 1977,55, 1033. Tai, C.; Dalby, F. W. Can. J . Phys. 1978, 56, 183. (2) Lehmann, K. K.; Smolarek, J.; Goodman, L. J . Chem. Phys. 1978.69, 1569. (3) Zandee, L.; Bernstein, R. B.; Lictin, D. A. J . Chem. Phys. 1978, 69, 3427. Zandee, L.; Bernstein, R. B. J . Chem. Phys. 1979, 71, 1359. (4) Miller, J. C.; Compton, R. N. J . Chem. Phys. 1981, 75, 22. J . Chem. Phys. 1981, 75, 2020. (5) McClelland, G. M.; Saenger, K. L.; Valenti, J. J.; Herschbach, D. R. J . Phys. Chem. 1979,83,947. (6) Aminov, A.; Evan, U.; Jortner, J. Chem. Phys. 1980, 51, 31-42. (7) Levy, D. H. Adu. Chem. Phys. 1981, 47, 323. (8) Miller, J. C.; Cheng, W. C. J . Phys. Chem. 1985, 89, 1647. (9) Miller, J. C.; Compton, R. N. J . Chem. Phys. 1986, 84, 675.
0022-3654/87/2091-2589$01.50/0
panding gas. Ions created by MPI were extracted and analyzed in a short (20 cm) time-of-flight mass spectrometer. The output of a channeltron detector was amplified, averaged in a boxcar integrator, and recorded. Room temperature iodine vapor (-0.5 Torr) was seeded into argon or helium at 1-5 atm. The gas mixture was expanded via a pulsed valve (Quanta-Ray) through a 0.5-mm orifice. The nozzle itself was sometimes heated to-70 OC to provide vibrationally warmer and also more intense I, spectra due to a higher concentration of Iz in the jet. Both the nozzle and laser were operated at 10 Hz and no skimmer was employed. The cell experiments were performed with a simple MPI cell utilizing two opposing wire electrodes. The cell was filled with neat I2 at its room temperature vapor pressure. The electron signal produced in the proportional counter was detected with a charge-sensitive preamp, then averaged, and displayed.
Results and Discussion Vibrational and Rotational Cooling. The vibronic spectra of I, expanded with argon in the supersonic jet is contrasted with a room temperature MPI spectrum of neat 1, in an MPI cell in Figure 1. The thermally equilibrated cell spectrum of Figure 1b is similar to those published previously',* with the exception of weak additional features to be discussed later. A seven-member progression (u'+ 0) is observed with an excited-state frequency, wo = 234 cm-'. The first hot band progression (u'+ 1) begins with a well-resolved origin -205 cm-' to the red of the 0-0 band and higher members appear as partially resolved side bands on the 0) high-energy side of the (0 0), (3 0), (4 0), and ( 5 bands. The intensity of the (0 1) band is approximately 70% 0) band. This intensity ratio includes conof that of the (0 tributions due to the Boltzmann distribution at 25 "C, FranckCondon (F-C) factors, and the wavelength-dependent intensity of the dye laser. Higher hot-band series (u'+ 2), (0'- 3), (u' 4), and (v'+ 5) are represented only by their first members which are successively red-shifted from the (0 0) band. The room temperature Boltzmann factors, normalized to the ground vibrational population, are 0.36, 0.13, 0.05, and 0.02 for the first four excited vibrations. The width of each band, which is larger than the laser bandwidth, represents a thermal rotational envelope. In contrast, the jet-cooled spectrum of the Dalby bands, shown in Figure l a , is clearly rotationally and vibrationally colder. The rotational envelopes are narrowed by about a factor of 3 and the vibrational hot bands are suppressed. The (0'- 1) progression is now well resolved and exhibits the two intensity maxima in the F-C distribution due to the node in the u" = 1 vibrational wave function. The intensity of the (0 1) band has dropped to about 25% of that of the origin. Since the wavelength dependence of the laser and the F-C factors have not changed, a vibrational temperature can be extracted. A value of Tvlb= 150 K reproduces the intensities of Figure l a reasonably well. In principle, rotational temperatures can be extracted by line shape a n a l y ~ i sbut , ~ in view of the limited laser bandwidth in these experiments such an analysis does not seem warranted. In experiments on nitric oxide
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The Journal of Physical Chemistry, Vol. 91, No. 10, 1987
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Figure 2. MPI spectrum of I2 in an argon expansion at a higher laser power than that of Figure 1 . TABLE I: Transition Energies and Vibrational Assignment for the New Band Svstem and the Dalbv Band Svstem of I,
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Figure 1. MPI spectra of (a) jet-cooled I2 in the molecular beam apparatus and (b) I, in a room temperature MPI cell.
expanded in argon under similar conditions, rotational temperatures of 1-5 K were obtained from rotationally resolved ~ p e c t r a . ~ Although I2 is not expected to be as rotationally cold as nitric oxide is similar to under similar conditions, it is assumed that the TI,, those reported p r e v i o ~ s l yfor ~ ~12/Ar ~ expansions under similar I 10 K. conditions of nozzle size and backing pressure, Le., TI,, In accord with previous results, the MPI mass spectrum is composed mainly of I+, with 12+observed weakly for the lower vibrational levels. The 12+/1+ratio decreases from its value at the origin band as the wavelength decreases (hence as u'increases). Furthermore, this ratio is power dependent. Earlier experiments reported 12+/1+ratios for the origin as about 0.5. In the present work a ratio of approximately 0.1 was observed. The power dependence is easily understood, as the production of 12+ is a three-photon process and production of I+ requires four photons. New Rydberg States. An inspection of both spectra of Figure 1 reveals weak unassigned peaks at -372, 374, and 375.5 nm. At higher laser power these peaks become more prominent and several additional peaks emerge from the background as shown in Figure 2. The additional peaks are easily sorted into a fivemembered vibrational progression (0' 0) with an average frequency of 238 cm-' and two hot bands, the (0 1) and (0 2), with approximately the ground-state I2 frequency. The electronic origin is thus assigned to the band at 377.2 nm, 571 cm-' to lower energy than the (0 0) band of the Dalby systems. These assignments are shown in Figure 2 and the positions of the new bands, as well as our measurements of the Dalby system, are given in Table I. As can be seen from the table and figure, the new band system is virtually identical with that of the Dalby bands with the exception of the red shift. That is, the ground- and excited-state frequencies are very similar as are the F-C distribution and the vibrational temperature. In spite of the stated goal of searching for van der Waals species the new band system must be assigned to monomeric I,. The new
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bands are present in approximdtely the same intensity relative to the Dalby bands under all preparation conditions. Iodine expanded with He, Ar, Ar/Kr, and Ar/Xe mixtures gave the same results as did introduction of neat I, at its vapor pressure. Figure 1b shows that the new bands are present in a room temperature MPI experiment with no buffer gas present. Consequently, any assignment to a 12-rare gas species is ruled out. The possibility of as the responsible species cannot be definitively ruled out but seems very unlikely. While a rare gas van der Waals partner typically produces only small changes in the spectroscopic constants, a major perturbation would be expected if another I, molecule were the partner. Gas-phase absorption bands attributed to (12)2 are diffuse and uncorrelated with the monomer spectra.1° Matrix isolation electronic and Raman spectra have also been attributed to dimers." The Raman data indicate a significant reduction of the ground-state vibrational frequency from 21 3 cm-' for I2 to 181 cm-I for (IJ2. While discrete matrix emission spectra were attributed to (12)2 the observed bands do not appear simply as perturbed monomer bands." The new bands are consequently assigned to a hitherto unknown Rydberg state of the I2 monomer. Rydberg states which share a common core configuration usually have very similar vibrational constants and F-C factors. The similarity of the hot band fre(10) Tamnes, M.; Duerksen, W. K.; Goodenow, J. M. J. Phys. Chem. 1968, 77, 966. Passchier, A. A,; Gregory, N. W. J . Phys. Chem. 1968, 72, 2691. (1 1) Howard, W. F.; Andrews, L. J. Raman Spectrosc. 1974, 2,447. Auk, B. S.; Andrews, L. J. Mol. Specfrosc. 1978, 70, 68.
Multiphoton Ionization of Jet-Cooled Iodine quency to that of the Dalby bands also supports the assignment to a common species. Two other experimental observations are relevant to the new band system. First, as mentioned previously, the Dalby bands appear primarily as I+ but can also be detected more weakly and with different intensity distribution as 1,'. The new bands are also detected via the atomic ion channel but no I,+ can be detected a t all. The 12+/1+branching ratio must at least be a factor of 5 smaller for the new bands. Second, the relative intensity of the Dalby and the new bands is laser power dependent. A room temperature cell spectrum similar to that of Figure 1b can be easily recorded with an unfocused laser, and the new bands are absent. Likewise, in the beam experiments, decreasing the laser power causes the new bands to disappear faster than the Dalby bands. The nonobservation of these bands in previous spectra's2 is presumably due to the lower laser power and longer focal lengths used in those experiments. In principle, this may mean that an additional photon is required for observation of these bands. However, the strong Dalby band system is saturated under the conditions for observation of the new system. Thus the laser power dependence probably indicates that the new transition is much weaker and is only observed at high power. A definitive assignment of the new band system is difficult based on the present data but a tentative assignment will be considered. The ground state of I2 is represented by the configuration uiI14,114 leading to the designation IZ,+(O,+). The ground state and valence excited states of I2 are discussed in detail by Mulliken.12 A two-photon transition in a homonuclear diatomic like I, demands a gerade excited state. The energy and vibrational constants of the new band suggest a Rydberg state assignment. Venkateswarlu13 has systematically analyzed the one-photon vacuum ultraviolet spectra and assigned several ungerade Rydberg series. He includes only two gerade states in his tabulation, but they do not fall in the present spectral region. The 12+ion has the configuration u:II:II: leading to 'II3/2,1/2, lowest states. The Dalby and Goodman band systems have been assigned as the states arising from a Rydberg electron in a u, orbital outside of the spin-orbit-split 'II3,2 and 2111j2ion core, respectively. Via analysis of the linear to circular photoionization ratio and the rotational envelope, the Dalby bands were assigned' as 1,(211,/,g)nsu,. The Goodman system was assigned2 as the spin-orbit partner of this state with the Rydberg electron having the same phase relationship to the core, hence the assignment as 2,(2113/2g)nsug.This latter assignment depended in part on the absence of additional bands in the region which would be expected from a 11, Rydberg electron. Such a II, assignment would include 2,, l,, l,, and 0, states with the 2111/2core and 3,, 2,, l,, and 0, with the 2 1 1 3 / 2 core.14 In particular, the 2, and two 1, states for the 211112 core should have similar excitation and ionization cross sections and be readily observed. As these extra bands were not observed, Lehman et aL2ruled out the n, assignment in favor of the 2(I13/2g)nsu,choice. The new band system observed here need not negate the above conclusions. The striking intensity difference between these bands and the Dalby system would rule out a possible assignment as the two 1, states of the (2111j2)II, assignment. Furthermore, an additional cluster of band systems has been observed at higher energy (discussed in the next section) which could be more appropriately assigned to the II electron states. Within the u, electron assignment for the Rydberg electron the new band system must be the 0,(2111j2)nustate, which is two-photon allowed from the ground O,+ state. The similarity of the electronic energy, vibrational constants, and F-C envelopes to those of the Dalby bands suggest that they share a common core assignment. The large intensity difference between the 1, and 0, states is attributed to different ionization rates. If these assignments are correct a single 1, state associated with the ('II3/2) ion core remains unobserved for the u, electron states. The present assignment to (12) Mulliken, R. S . J . Chem. Phys. 1971, 55, 288. (13) Venkateswarlu, P. Can. J . Phys. 1970, B48, 1055. (14) Herzberg, G. The Spectra of Diatomic Molecules; Van NostrandReinhold: New York, 1950; 2nd ed, Chapter VI.
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an 0, state could in principle be tested by polarization measurements. However, the strong saturation of the Dalby bands and the nonobservation of the new bands at lower power makes polarization measurements difficult. Other Band Systems. To higher energy than the Dalby bands, a cluster of additional new transitions are observed between 345 and 355 nm. These bands are shown in Figure 3 for the case of (a) jet-cooled I, and (b) room temperature 1, in an MPI cell. No two-photon spectra have previously been observed in this region nor are any states known from traditional spectroscopy. The present results are considered preliminary and are not well understood. Nevertheless, a number of qualitative points can be made. The cell spectrum is relatively simple and corresponds to two quite similar electronic bands labeled A and A' in the figure. Both bands have vibrational spacings of about 225 cm-' and similar F-C envelopes. Their origins are separated by 871 cm-I. These two systems thus probably have the same configuration. These states probably arise from a II, Rydberg electron outside of one of the 211,/2,3j2ion cores. A reasonable assignment might be to
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The Journal of Physical Chemistry, Vol. 91, No. 10, 1987
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the two 2, states arising from a (211112)11,configuration; however, of a potential (12-RG)+ ion to any charged product such as I,+, it is clearly premature to make any firm assignment. I+, or RG+ would be detected via the observation of these daughter The beam spectrum of Figure 3a, in contrast, is considerably ions. The mass-resolved excitation spectrum of the final product more complicated. The A and A' states observed in the cell spectra ion would then reveal spectra of the neutral van der Waals can be found but their intensity distribution has been changed. molecule. This is an old problem in MPI spectroscopy where In addition, two new band systems are observed with somewhat laser-induced fragmentation often precludes observing the parent higher vibrational frequencies. These are labeled B and B'. (These ion at all. However, excitation spectra recorded while detecting Rydberg spectra labeled B and B' should not be confused with any daughter ion usually lead to a faithful representation of the the valence B 3110+ustate.) The spectrum was obtained by neutral parent molecule spectrum. For instance, 1, is detected time-gating the time-of-flight signal at the If mass, thus ruling as 1+,3-4 benzene as Cf (or any sized fragment from Cf to out impurities. The B, B' bands are observed in both argon (as C6H6+),'*and, more to the point, NOAr can be detected as NO+: shown in the figure) and helium expansions, thus eliminating and NOXe as Xef.19 Many other examples could be named. possible dimer assignments. The spectrum of the He/12 expansion Hence, we conclude that 12-RGf is not being formed at all and is similar to that of Figure 3a except that the bands are broader, the destruction must occur in the neutral molecules. reflecting a higher rotational temperature. In addition, the relative Some of these pathways can be ruled out also. If a van der intensities of the A, A' and B, B' bands are somewhat changed. Waals species dissociates to neutral I, in an excited state that is Comparable power densities and focusing conditions were used within one photon of the ionization limit, it could also be detected. for both the cell and beam experiments. These spectra are difficult For instance, NOAr in the D state dissociates into ground-state to rationalize and more experimental work is required in order argon plus nitric oxide in the C state, from which it is subsequently to understand them. In particular, a search to still higher energy ionized and detected as NO+. For excitation into a state of the should reveal more members of the II, cluster of states. Also, complex associated with the Dalby bands of I2 any dissociation polarization measurements will help in this assignment. into that Rydberg state of I, would thus still be detected as 1', Rare Gas-I, van der Waals Species. As stated in the Introor I+. Vibrational predissociation of a v'level of I,-RG into a duction, the original intent of the present investigation of suv'- 1 level of I,, as has been extensively studied for the B state, personically expanded iodine was to search for 12-rare gas van can be ruled out here. Furthermore, the u = 0 level, which could der Waals species. These species have been well characterized not dissociate via this mechanism, should be observed. Hence, by laser-induced fluorescence following excitation into the B 3110+u for excitation of the Dalby bands the cluster destruction must occur valence state.' The MPI technique is ideally suited for study of by predissociation to lower-lying excited states of I, from the Rydberg states and several such states of nitric oxide-rare gas two-photon level or via predissociation of possible resonant inspecies have been recently c h a r a ~ t e r i z e d .However, ~ ~ ~ ~ in spite termediates at the one-photon level. N o states of I, have been of the observation of several new band systems in the present work, well characterized in this region (360-375 nm) but studies of the none can be assigned to the van der Waals molecules. Comparison cage effect in complexed I, indicates that appreciable absorption with other studies which used similar expansion c ~ n d i t i o n s ~ . ' ~ J ~of the complex occurs at somewhat lower energies (-450 nm).15J6 leads to the conclusion that these complexes are present in our As ionization can often compete with predissociation,8 the proposed expansion. Certainly, they should be formed in numbers similar destruction of I,-RG must be extremely rapid, perhaps occurring to the nitric oxide-rare gas species studied under the same convia a purely repulsive state. Possible means of detecting such ditions.* With the signal-to-noise available in the present exelusive species with MPI techniques would include two-laser experiments, 1,-RG molecules could be detected at the 0.5% level periments (such as those performed earlier on iodine20) in order in the 12+ mass channel and perhaps two orders of magnitude to eliminate dissociative states or the use of picosecond lasers. smaller concentration could be detected in the 12-RG+ mass In summary, for a ( 2 1) multiphoton ionization scheme there are three possible states where dissociation can occur. Dissociation channel where there is no background signal. Mass resolved (2 1) MPI is capable of sub-ppm detection of minor beam conof 12-RG+ can be ruled out as can vibrational predissociation of s t i t u e n t ~ . In ~ ~principle, observation of the dimer via the B 3110+u 1,-RG at the two-photon level. Consequently, either the twovalence state, where the spectra have been well characterized by photon state must rapidly dissociate to a low-lying state (more LIF techniques, would establish the concentration. Unfortunately, than one photon to the ionization potential) or the destruction must occur at the nominally virtual one-photon level. Of course, it must direct probing of the B state via MPI in search of the 12-RG species is difficult due to the several additional virtual levels be first be demonstrated that such molecules are, in fact, present encountered on the way to ionization., However, if there are in our beam. dimers present in the beam (as assumed but not demonstrated) Acknowledgment. Research sponsored by the Office of Health then the failure to observe their Rydberg spectra probably indicates and Environmental Research, U S . Department of Energy under a rapid destruction mechanism. contract DE-ACO5-840R21400 with Martin Marietta Energy Although there are a number of possible dissociation pathways, Systems, Inc. several can be ruled out quite easily. For instance, the dissociation
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(15) Valentini, J.; Cross, J. B. J . Chem. Phys. 1982, 77, 572. (16) Saenger, K. L.; McClelland, G. M.; Herschbach, D. R. J . Phys. Chem. 1981,85, 3333. (17) Miller, J. C. Anal. Chem. 1986, 58, 1702.
(18) Cooper, C. D.; Williamson, A. D.; Miller, J. C.; Compton, R. N. J Chem. Phys. 1980, 73, 1527. (19) Miller, J. C. J . Chem. Phys., in press. (20) Williamson, A. D.; Compton, R. N . Chem. Phys. Lett. 1979,62,295.
