T. G. Dietz

Laser production of supersonic metal cluster beams

Cold beams of metal clusters are produced by combining a laser vaporization technique with pulsed supersonic nozzle technology. (AIP)

Two‐color photoionization of naphthalene and benzene at threshold

We observe the resonant two‐photon ionization (R2PI) spectrum of naphthalene and benzene at and just above the first ionization potential in a supersonic molecular beam. For naphthalene, the spectrum consists of direct ionization step functions indicating the relative Franck–Condon factors for transitions into the ground state ion. The spacings observed between these steps measure the ν8 vibrational frequency in the naphthalene ion. The electric field dependence of these steps is found to be consistent with a field ionization red shift of the thresholds. In benzene we observe similar direct ionization. Superimposed on the threshold steps, however, are broad features (∼100 cm−1) due to vibrational autoionization of Rydberg states at this same energy. Implications of this data on the observation of high Rydbergs in such large organic molecules are discussed.

Resonance enhanced two‐photon ionization studies in a supersonic molecular beam: Bromobenzene and iodobenzene

The 1B2(ππ*)←1A1 absorptions of two monosubstituted halobenzenes have been investigated using resonance enhanced two‐photon ionization in a pulsed supersonic molecular beam. Detection of the photoions was accomplished by means of a time‐of‐flight mass spectrometer. The 1B2←1A1 system of bromobenzene has been observed with good sensitivity using this technique, even though the total decay rate of the 1B2 state is greater than 1×1011 sec−1. No ion signal was observed when the same transition was probed in iodobenzene, allowing us to place a lower limit on its decay rate of 4×1013 sec−1.

Efficient multiphoton ionization of metal carbonyls cooled in a pulsed supersonic beam

Abstract Laser excitation of Fe(CO)5, Cr(CO)6 and Mo(CO)6 in the region of the charge transfer bands around 2800 A is found to produce Fe+, Cr+ and Mo+ with near unit efficiency in a laser flux as low as 108 W cm−2. The uncomplexed metal ion continues to dominate the photoion yield down to near threshold at a laser flux of 3 × 105 W cm−2. Excitation spectra of this photoion production indicate the absorption in this region is intrinsically diffuse.

Efficient multiphoton ionization of jet-cooled aniline

Abstract Laser excitation of the S 1 ← s o transition in super-cold beams of aniline is found to produce the parent ion with nearly 100% efficiency at a laser flux as low as 2 × 10 7 W cm −2 . Excitation spectra of the photoion signal are similar to the known S 1 ← So absorption spectrum with the addition of power broadening and a non-resonant coherent background.

Time evolution studies of triplet toluene by two‐color photoionization

Two‐color photoionization has been applied to measurement of the time evolution of laser‐excited 1B2(ππ*) vibronic levels of toluene. The observed photoionization signals exhibit a biexponential dependence upon the time delay between the pump and ionizing lasers. This photon intensity is shown to be proportional to the population of the excited singlet (S1) and triplet (T†) electronic states. The data is fit to a simple kinetic model which yields the rates of S1 fluorescent decay, S1→T†, and T†→S‡0 intersystem crossings. The measured quantum yield for intersystem crossing was found to agree with previous work. The triplet lifetimes measured are considerably shorter than values reported for collisionally relaxed triplets states due to the steep excess vibrational energy dependence of the nonradiative rate. This excess energy dependence of the triplet decay rate has been measured over a limited range of energy and the observed rates are found not to extrapolate smoothly to the decay rate measured for the vi...

New vibronic bands of CH2 observed in a pulsed supersonic jet

Abstract Spectroscopic studies of the methylene radical in a supersonic expansion have resulted in the observation of previously unreported CH 2 B 1 B 1 ← a 1 A 1 , hot band transitions. These a state levels are populated by radiative cascade following multiphoton dissociation of ketene. Multiphoton excitation of ketene is also found to produce a diffuse luminescence with a lifetime longer than 50 μs. The conclusion of Lengel and Zare are singlet methylene is not produced one-photon nitrogen laser photolysis of cold ketene is confirmed.

Laser synthesis of metal clusters from metal carbonyl microcrystals

One common preparative method for multinuclear metal clusters is irradiation by sunlight. Preparation of Fe/sub 2/ (CO)/sub 9/ in this way is limited, however, by metal-metal bond scission. It was found that intense pulsed ArF excimer laser irradiation produces a much more favorable situation. At 20 mJ cm/sup -2/, a 10-ns ArF laser pulse will excite Fe(CO)/sub 5/ molecules at a rate of roughly 2 x 10/sup 8/ s/sup -1/. Halving this fluence from 20 to 10 mJ cm/sup -2/ reduced the Fe/sub x/ cluster intensity by a factor of 20. (DLC)

Spectral narrowing and infrared laser fragmentation of jet‐cooled UO2(hfaa)2 TMP and UO2(hfaa)2 THF: Volatile uranyl compounds

The temperature dependence of the CO2 laser‐induced decomposition spectral linewidths and yields for jet‐cooled UO2(hfaa)2 TMP and UO2(hfaa)2 THF molecules has been studied using laser‐induced fluorescence detection and photoionization time‐of‐flight mass spectrometry on a pulsed supersonic beam of uranyl molecules seeded into a helium carrier gas. The two uranyl species, although chemically very similar, show a remarkable difference in the temperature dependence of the linewidth. For UO2(hfaa)2 TMP, the linewidth decreases upon cooling by a factor of ∼15, while the UO2(hfaa)2 THF linewidth decreases by only a factor of ∼2.6 over the same temperature range. The differences in the linewidth behavior are reflected in the yield curves for the laser‐induced reaction for the cooled molecules. These results for UO2(hfaa)2 TMP, to our knowledge, represent the first measurements of the temperature dependence of a T2 line broadening process for an isolated gas phase molecule. Detection of beam molecules as well as...