2250
J . Phys. Chem. 1984, 88, 2250-2255
briefly this photolytic process by using various color filters in the xenon lamp to establish the wavelength region effective in producing Cu02 Such experiments have shown that even a prolonged irradiation through filters cutting off radiation at wavelengths shorter than -2400 A resulted in little observable product. On the other hand, unfiltered Xe arc radiation led to rapid growth of the Cu02signal, in spite of the fact that the lamp has only very little output between 2500 A and the air cutoff. Upon photolysis with the unfiltered xenon arc, the Cu02signal initially increases linearly with time, but the growth levels off very rapidly. After -5-10 min of photolysis the further growth of the Cu02 emission intensity is very slow. At that point, the Cu atoms and O2molecule reactants are far from being depleted and, in fact, absorption spectra show only an insignificant decrease in Cu atom concentration. We conclude that C u 0 2 is efficiently produced by excitation into the Schumann-Runge region of those O2molecules which have a Cu atom nearby (possibly Cu-O2 van der Waals complexes). On the other hand, excitation in the visible and near-UV regions and, in particular, in the region of the resonant 4s2S 4 p 2 P transition of copper atoms results in little -+
product under our conditions. The quantum yield of Cu02 formation following 4p2P Cu excitation must be low. This result conflicts somewhat with the findings of Mitchell et a1.,I0 who generate Cu02by Cu atom excitation at 3300 A and who conclude that the reaction can be carried to nearly complete depletion of Cu atoms. The main differences may be the 1-kW arc lamp used by them (as opposed to the 50-W one used in our experiments) and the much higher concentrations employed (1:lOO) in their study. Summary The reactions of oxygen with copper atoms in solid neon and argon are examined. The recent suggestion by Tevault that the strong visible fluorescence observed in matrices containing copper should be reassigned to Cu02 is confirmed. We show more specifically that CuO, is linear and centrosymmetric and report its vibrational frequencies. The electronic structure and formation of the molecule are discussed. Registry No. CU'~O~, 89087-86-5;Cu20,1317-39-1;CuO,1317-38-0; CU,7440-50-8;0 2 , 7782-44-7;"02, 14797-71-8.
Two-Color Laser Photoionization Spectroscopy in a Collisionless Free-Jet Expansion: Spectroscopy and Excited-State Dynamics of Diazabicylooctane Mark A. Smith, James W. Hager, and Stephen C. Wallace* Department of Chemistry, University of Toronto, Toronto, Ontario M5S I A l , Canada (Received: November 18, 1983)
The photoionization efficiency spectra of 1,4-diazabicyclo[2.2.2]octane(Dabco), in a supersonicjet, have been obtained from the excited Rydberg states 'E' (3p,(+)) and 'Al' (3s(+)) by using two-color laser photoionization spectroscopy. Precise adiabatic ionization thresholds are observed, with a lack of observable high-lying Rydberg structure due to the cooling and collisionless properties of the jet. The adiabatic ionization potential (IP) is determined to be 58 052 f 5 cm-'. Efficient internal conversion is observed from the intermediate 'E' ground vibrational state to the 'Al' and 'A; (3s(-)) states. Though fluorescenceis not observable from the 'E' state, a small residual population in this level is directly observed via photoionization and is found to decay at a different rate from the 'Al' levels. From the enhanced sensitivity of this free-jet expansion photoionization technique, a comprehensive picture emerges for the photophysics of Dabco in the 'E' state.
Introduction Recent studies have shown that two-color resonance-enhanced photoionization spectroscopy can be a very sensitive method to probe the higher excited states as well as the low-lying ionic states of polyatomic molecules.'" In these experiments light at one wavelength (XI) prepares the molecular system into a specific excited electronic state. Shortly thereafter, light at a second wavelength (A,) ionizes the excited-state species. By tuning the first wavelength, the second remaining sufficiently short to ionize any excited molecules, ions are produced only when XI comes into resonance with an excited level. This experiment promises to be a useful alternative to the laser-induced fluorescence technique, especially for molecular systems with either very short or very long excited-state lifetimes. Variation of the delay between the arrival of XI and X2 can be used to directly determine the intermediate state lifetime yielding information on the excited-state dynamics of the molecular species. An important extension in
two-color photoionization studies is to hold XI fixed, tuned to a vibronic level in a given excited electronic state, and then tune the second wavelength through the ionization continuum to yield photoionization efficiency curves for the excited states.&* In these latter experiments, the intermediate excited state can have (especially for the case of Rydberg states) greatly enhanced Franck-Condon factors with the lower ionic levels compared to the ground state. This leads to much more intense and sharper initial t h r e ~ h o l d scompared ~,~ to one-photon ionization techniques. This approach, using two-color laser photoionization spectroscopy with two tunable lasers, can overcome many of the problems inherent with conventional techniques of ionization spectroscopy.9-'2 Another serious impediment in the spectroscopy of large polyatomic molecules has been the large number of vibrational bands populated under room temperature conditions and the broad rotational envelopes encompassing them. The coupling of free-jet
(1) D. H. Parker and M. A. Al-Sayed, Chem. Phys., 42, 379 (1979). (2) A. D. Williamson, R. N. Compton, and V. H. D. Eland, J . Chem. Phys., 70, 590 (1979). (3) M. A. Duncan, T. G. Dietz, and R. E. Smalley, J. Chem. Phys., 75, 2118 (1981). (4) G. J. Fisanick, T. S. Eichelberger IV, M. B. Robin, N . A. Keubler, J. Phys. Chem., 87, 2240 (1983). (5) K. H.Fung, W. E. Henke, T. R. Hays, H. L. Selze, and E. W. Schlag, J . Phys. Chem., 85, 3560 (1981). (6) M. A. Smith, J. W. Hager, and S. C. Wallace, J . Chem. Phys. in press.
(7) N. Gonohe, N. Yatsuda, N. Mikami, and M. Ito, Bull Chem. SOC. Jpn., 55, 2796 (1982). (8) M. Fujii, T. Ebata, N. Mikami, and M. Ito, Chem. Phys. Lett., 101, 578 (1983). (9) J. Berkowitz, "Photoabsorption, Photoionization and Photoelectron Spectroscopy", Academic Press, New York, 1979. (10) C. Y. Ng in "Advances in Chemical Physics", Vol L11,I. Prigogine and S . A. Rice, Eds., Wiley, New York, 1983, p 263. (11) P. M. Johnson, Acc. Chem. Res., 13, 20 (1980). (12) D. H. Parker, J. 0. Berg, and M. A. El-Sayed in "Advances in Laser Chemistry", A. H. Zewail, Ed., Springer, Berlin, 1978,p 320.
0022-3654/84/2088-2250$01.50/00 1984 American Chemical Society
Two-Color Laser Photoionization Spectroscopy ENERGY
STATE
I
IO 000
0
t
SELECTION RULE FROM X'A; (n-PWTON)
SHG
Nd YAG L A S E R
CRYSTAL
EXCIMER
Figure 2. Schematic diagram of the laboratory setup used to obtain one-color multiphoton ionization spectrum (2X,) and two-color photoionization efficiency spectra of jet-cooled Dabco. -
X 'A;
Figure 1. Dabco excited-state energies and optical selection rules from the ground state. The solid lines indicate observed origin (0-0) bands. The dotted lines indicate calculated origin positions. The hatched regions indicate observed diffuse absorption assigned to these states. See text for references.
expansion technology with laser spectroscopic techniques has proven to be an invaluable method of simplifying these electronic spectra. Thus, it is possible to produce vibrationally and rotationally cooled molecules whereby hot-band structure can be virtually eliminated and rotational envelopes can be collapsed to yield vibronic bandwidths approaching the resolution of conventional pulsed laser systems. This leads to both an increase in sensitivity as well as greater precision in the determination of thresholds in photoionization spectroscopy. A further advantage of using a free-jet expansion in these studies appears to arise from the collisionless properties in the postnozzle region where the laser excitation occurs. In the absence of collisions, the very high-lying Rydberg states in and around the ionization continuum cannot contribute significantly to the observed ionization signal. This further simplifies the threshold spectra since there are no overlaying Rydberg resonances on the threshold as is generally seen in cell photoionization s t u d i e ~ . ~The ~ ~ ionization J~ potential can then be accurately and directly obtained from the threshold onset and does not depend upon the inherent problems of assigning the Rydberg series leading to the continuum. The spectroscopy of the lower electronic levels of Dabco has been the topic of experimental and theoretical ~ t u d i e s . ' ~ - The '~ nature of the symmetry of the molecule (D3&with its two identical yet isolated nitrogen atoms has increased our understanding of through-space interactions in molecular systems. The ordering of the first three electronic states has only recently been unambiguously res01ved.l~ As of yet, all of the observed states are Rydberg in nature. These Rydberg levels are delocalized over both nitrogen atoms composed of symmetric and unsymmetric combinations of the isolated Rydberg orbitals located on each N atom. The symmetrical combination is found to lie at lower energy. Figure 1 shows schematically what is presently known concerning these levels and their assignments. The lowest state, lAl' (3s(+)), is a strongly two-photon-allowed state and has been seen in fluorescence as well as in two-color (13) A. M.Halpern, J. L. Roebber, and K. Weiss, J . Chem. Phys., 49, 1348 (1968). (14) (a) A. M. Halpern and T. Gartman, J . Am. Chem. Soc., 96, 1393 (1974); (b) A. M. Halpern, ibid., 96, 7655 (1974); (c) A. M. Halpern and P. P. Chan, ibid., 97, 2971 (1975). (1 5) A. M. Halpern, Chem. Phys. Lett., 6, 296 (1 970). (16) Y . Hamada, A. Y . Hirakawa, and M. Isuboi, J . Mol. Spctrosc., 47, 440 (1973). (17) P. Avouris and A. Rossi, J . Phys. Chem., 85 2340 (1981), and ref-
erences therein.
The Journal of Physical Chemistry, Vol. 88, No. 11, 1984 2251
photoionization ~ t u d i e s . ~The ~ ~origin ~ ~ ~ *of the next level, 'Ai', has not yet been observed. The term value has been ~ a l c u l a t e d ' ~ to lie between 37 000 and 39 000 cm-' and a weak continuum in cell absorption studiesI3 in the region 37 400 to 39 000 cm-' has been tentatively assigned to this state.I7 The 'E' state is both oneand two-photon allowed from the lAl' ground state and has been studied via cell fluorescence s t ~ d i e s . ' ~ , ' The ~ , ' ~fluorescence shows an unusually large Stokes shift (-4000 cm-') which has been suggested to be due to fast internal conversion of the 'E' state to the 'Al' state, from which essentially all of the fluorescence ari s e ~ . ' * ~This ~ , ' explains ~ the long lifetise (-800 ns) associated with the fluorescence as the lAl' X'A,' fluorescence is one photon forbidden in first order. The photoelectron spectrum of Dabco yields an assignment of the ionization potential at 7.15 eV; however, this is uncertain due to the low intensity of the adiabatic transition.lg A regular progression at 750 cm-' is observed. Parker and El-Sayed' have reported an initial study of the two-color photoionization of Dabco from the 'E' state, yielding a crude threshold spectrum but demonstrating the high sensitivity of this technique. More recently, Fisanick and co-workers4 and Ito and co-workers7~* have reported studies of the two-color photoionization spectrum of Dabco using the 'Al' resonance and some vibrational information on the D a h radical cation. In this report, we have investigated the photoionization efficiency spectra of Dabco from two electronically excited states, the 'E' (3p,(+)) and 'Al' (3s(+)) Rydberg levels. For the 'E' (0-0) level, we have recorded the photoionization threshold spectrum 6000 cm-' into the ionization continuum. From this we assign an ionization potential of rotationally cooled D a h (
