J. B. Hopkins

Supersonic metal cluster beams of refractory metals: Spectral investigations of ultracold Mo2

A novel technique involving pulsed laser vaporization of the bulk metal within a pulsed supersonic nozzle has been shown to successfully produce ultracold bare metal clusters of even the most refractory of metals, tungsten and molybdenum. Clusters of up to 25 atoms may be readily prepared using this technique. Mass‐selective resonant two‐photon ionization spectra of Mo2 produced in this fashion show that the dimer is efficiently cooled in the expansion Ttrans<6 K, Trot∼5 K, and Tvib∼325 K. We have rotationally resolved the A 1Σ+u←X 1Σ+g (0–0) band for 92Mo2 and determined the bond length in the ground and excited states to be 1.940±0.009 and 1.937±0.008 A, respectively. This confirms and extends the analysis of Efremov et al. [J. Mol. Spectrosc. 73, 40 (1970)] who prepared 98Mo2 by flash photolysis of isotopically pure Mo(CO)6. We have also observed the (1–1), (2–2), and (3–3) sequence bands which together with the ground state data of Efremov et al. determine vibrational constants ω′e=449.0±0.2 cm−1 and ...

Vibrational relaxation in jet‐cooled alkyl benzenes. I.Absorption spectra

Fluorescence excitation spectra of a series of alkylbenzenes cooled in a supersonic free jet have been obtained for the first 1000 cm−1 region of the 1B2(ππ*)←1A1 ultraviolet absorption spectrum. The series includes all n‐alkylbenzenes up to n‐hexyl together with isopropyl‐ and tert‐butylbenzene. As with toluene, the spectra in this region for all alkylbenzenes is found to be dominated by vibrations of five ring modes: 6a, 6b, 1, 12, and 18a. Three of these—the ’’system modes’’ (6b, 12, 18a)—are found to be largely invariant to changes in the alkyl chain length and type— the ’’bath.’’ For n‐alkylbenzenes with chain length of three or higher, spectra of two distinct conformations are observed with roughly equal intensity. These conformations are distinguished by configuration about the 1–2 carbon–carbon bond of the alkyl chain. When this configuration is trans [n‐(t)‐alkylbenzene], the alkyl chain extends away from the phenyl ring leaving the ring free to van der Waals complex binding on both sides of the ...

Jet‐cooled naphthalene. I. Absorption spectra and line profiles

Fluorescence excitation spectra have been recorded for the first 6000 cm−1 of the ultraviolet spectrum of h8‐ and d8‐naphthalene cooled in a supersonic free jet. Measured profiles of vibronic bands in these spectra display a monotonically increasing width as a function of vibrational energy (Ev) in the excited electronic state. The high fluorescence quantum yield and relatively constant fluorescence lifetimes in these spectral regions require the measured line broadening to be assigned to intramolecular vibrational relaxation (IVR) within the excited electronic state. The rate of this IVR process as measured by the width of the broadened profiles increases smoothly from 9×1010 sec−1 at Ev=3068 cm−1 to 7×1011 sec−1 at Ev=5200 cm−1 for h8‐naphthalene. Line profiles of d8‐naphthalene in this same spectral region are found to be ∼50% wider.

Vibrational relaxation in jet‐cooled alkyl benzenes. II. Fluorescence spectra

Dispersed fluorescence spectra of a series of laser‐excited n‐alkylbenzenes cooled in a supersonic free jet have been obtained under effectively collision‐free conditions. The series includes all members of the n‐alkylbenzene family from methyl‐ through n‐hexylbenzene. Pulsed laser excitation was cleanly made into the 000, 6b01, 1210, and 18a10 vibronic bands of the 1B2(ππ*)←1A1 ultraviolet absorption spectrum. Resonance fluorescence from the O00 excitation experiments permitted clear assignment of the six ring modes (6a, 6b, 1, 12, 18a, 9a) which in combinations and short progressions dominate the fluorescence spectra of these molecules. Except for 6a and 1, the frequency of these ground state vibrations is found to be constant throughout the alkylbenzene series to better than 1 part in 100. Fluorescence from the 6b10, 1210, and 18a10 excitation experiments with alkylbenzenes of successively longer chains showed clearly the onset and increasing importance of intramolecular vibrational relaxation (IVR) oc...

Jet‐cooled naphthalene. II. Single vibronic level fluorescence spectra

Single vibronic level fluorescence spectra have been measured for naphthalene cooled in a supersonic free jet. Ten spectra in all have been recorded, the excitation energies ranging from 0 to 4029 cm−1 above the 1B3u(ππ*)←1Ag origin. These results reveal extensive intramolecular vibrational redistribution (IVR) occurring in the absence of collision on a time scale much faster than fluorescence even at vibrational energies as low as 2570 cm−1. Comparison of the observed spectra with model calculations shows that this IVR process involves most of the harmonic overtone/combination basis states of overall b1g symmetry having energies within the absorption line profile.

Vibrational relaxation in jet‐cooled para‐alkylanilines

Absorption and fluorescence spectra have been obtained for the 1B2(ππ*)←1A1 transition of a series of para‐alkylanilines cooled in a supersonic free jet. The series consists of all p‐n‐alkylanilines through n‐butyl‐ as well as p‐isopropylaniline and aniline itself. For those members of the series having an alkyl chain length of three or greater, two distinct molecular conformations are found to be populated in the jet with roughly equal likelihood. Spectra of both the extended chain conformation [p‐n‐(t)‐alkylanilines] and the coiled chain conformation [p‐n‐(g)‐alkylanilines] show that two optically active aniline vibrations are largely unaffected by changes in the length and nature of the n‐alkyl chain. One of these vibrations (the inversion motion of the –NH2 group) would be expected to be particularly weakly coupled to the torsions and bends of p‐alkyl chain. Nonetheless, fluorescence spectra obtained when this vibration is excited in the 1B2 state show that intramolecular vibrational relaxation occurs...

Vibrational relaxation in jet‐cooled alkyl benzenes. III. Nanosecond time evolution

Spectrally resolved fluorescence time decay measurements have been completed with nanosecond resolution on a series of n‐alkylbenzenes laser excited into well‐localized ring distortion vibrations in the S1 electronic state. Results indicate that those early members of the series which continue to exhibit some sharp, vibrationally unrelaxed emissions do so because they are intermediate case examples with inadequate density of states to permit dynamical relaxation on a nanosecond time scale. The longer chain, statistical limit molecules show a relaxed fluorescence pattern which displays no residual nanosecond time evolution. Intramolecular vibrational relaxation thus appears to proceed to an essentially complete randomization within a time period shorter than the excitation laser pulse.

Vibrational relaxation in jet-cooled phenylalkynes

Absorption and dispersed fluorescence spectra are reported for a variety of 1‐phenylalkynes cooled in a supersonic free jet. The 1B2(ππ)←1A1 spectra of these compounds is characterized by a near perfect cancellation of transition strength sources for the totally symmetric vibronic bands including the origin. Two skeletal distortion modes are found to dominate the spectra, inducing strong transitions by vibronic coupling to a higher state of 1A1 symmetry. These two modes are rotated in the 1B2 state relative to their configuration in the ground state. The nature of these modes is found to be largely unaffected by changes in the length of the alkyl chain, but vibrational relaxation into the chain still appears to proceed on a subnanosecond time scale.

Predissociation probes of vibrational energy localization

Ring‐bound van der Waals complexes of the alkylbenzenes are studied.(AIP)Ring‐bound van der Waals complexes of the alkylbenzenes are studied.(AIP)

Ground state vibrational randomization in alkylbenzenes

Measurements of intramolecular vibrational randomization (IVR) previously carried out on the S 1excited state of a series of jet‐cooled alkylbenzenes have been extended to the ground electronic state. The ground state ring modes 6b, 1, and 12 were conveniently activatd by pulsed laser excitation of the 00 0 vibronic band of the S 1←S 0 ultraviolet absorption system, followed by fluorescence. The onset of IVR was then probed by resonant two‐photon ionization (R2PI) of these vibrationally activated ground state alkylbenzenes. When the alkyl chain exceeded three carbon atoms in length, the hot band 120 1 (E v ∼1010 cm− 1) was found to be absent from the R2PI‐probed spectrum, while the 6b 0 1 hot band (E v ∼625 cm− 1) remained easily observable through n‐pentylbenzene. This overall pattern of IVR is essentially the same as that observed previously in the S 1excited state. In both electronic states, the only effective barrier to vibrational energy decay from the ring to the chain appears to be an inadequate density of states. Above the critical state density, IVR appears to be complete, and this critical state density appears to be about the same in both the ground and excited electronic states.