R. F. Curl

Photophysics of buckminsterfullerene and other carbon cluster ions

The laser-induced fragmentation behavior of positive carbon cluster ions has been investigated by tandem time-of-flight techniques for the jet-cooled clusters up to 80 atoms in size. Two distinct photophysical regimes were found. The first applies to clusters with 34 atoms or more, all of which dissociate to produce even numbered fragments. Large even clusters fragment by the loss of the high energy species C2, odd ones lose a C atom. The second regime applies to clusters composed of 31 or less atoms, all of which fragment by the loss of C3. These two regimes are sharply separated by C + 32 which fragments to produce small cluster ions in the 10–19 atom size range. Fragmentation of the large clusters occurs on a microsecond or faster time scale only at very high levels of excitation (>12.8 eV). These photophysical results are interpreted as consequences of the large even clusters having edgeless, spheroidal cage structures while the small ones have linear chain or ring structures.

Supersonic cluster beams of III−V semiconductors: GaxAsy

Supersonic beams of semiconductor clusters with the formula GaxAsy were generated by laser vaporization of a disc of pure GaAs mounted on the side of a pulsed supersonic nozzle. These cluster beams were characterized by laser photoionization with various fixed-frequency lasers followed by time-of-flight mass spectrometry. Mass analysis of the clusters with x+y>10 showed all clusters in the composition range from Gax+y through GaxAsy to Asx+y to be present in roughly the amount expected from a binomial distribution. In the smaller clusters strong variations were observed from this expected binomial distirbution as a result of kinetic effects in the cluster formation process. Photoionization with an ArF excimer laser at very low pulse energy revealed a pronounced even/odd alternation in the photoionization cross section of the GaxAsy clusters, depending only on the total number of atoms in the cluster. Clusters in the 5–21 atom range with an odd number of atoms were one-photon ionized by the 6.4 eV ArF excimer laser photons. This even/odd alternation in ionization properties of the clusters supports the view that the even clusters have fully paired singlet ground states with no dangling bonds. At higher ArF excimer laser fluences, the observed mass spectrum became increasingly affected by fragmentation. As is true with bulk GaAs surfaces, these GaxAsy clusters evaporate largely by the loss of arsenic (probably As2) when heated by the laser, leaving behind clusters which are richer in gallium.

The relationship between electron spin rotation coupling constants and g-tensor components

The Hamiltonian (Pauli approximation) which gives rise to the spinrotation coupling tensor and the electron spin g tensor is set up for a 2Σ polyatomic molecule. A relationship between the spin-rotation interaction and the Zeeman effect is suggested. The magnitudes of those terms which prevent this relationship from being exact are estimated. The relationship should be valid to two significant figures in the correction of the g factor from that of the free electron in most cases. Some comparisons with experiment are made.

Microwave Spectrum of NO2: Fine Structure and Magnetic Coupling

Some 69 microwave absorption lines from three isotopic modifications of NO2 have been observed and identified. This spectrum has been completely analyzed in terms of moments of inertia, magnetic fine structure, and centrifugal distortion. The early stages of analysis were complicated by the existence of many mathematical solutions satisfying all of the data then available, but special methods have been developed for treating limited data and selecting the one solution giving the true molecular parameters.The results obtained for 14N16O2 are: Rotational constantsA=239 868.7±11 Mc/secB= 13 000.12±0.6 Mc/secC= 12 303.45±0.6 Mc/secOdd electron expectation values with respect to〈(3ra2−1)/2r3〉=−1.73×1024cm−3N−nucleus〈(rb−irc)2/r3〉=+3.34×1024cm−3|ψ(0)|2=+3.065×1024cm−3Reduced spin−rotation coupling constantseaa/A=+0.022563ebb/B=+0.000608ecc/C=−0.007764Force constants from combined microwave−infrared datafd=11.043±0.05  mydn/Afα= 1.109±0.01fdd=0.481±0.014fdα=2.140±0.05. The magnetic coupling constants are disc...

Photodetachment and photofragmentation studies of semiconductor cluster anions

Silicon, germanium, and gallium arsenide negative cluster ions are produced by laser vaporization followed by free supersonic expansion. Electron affinities (EA) of the corresponding neutral clusters are roughly bracketed by measuring the fluence dependence for photodetachment from anions at discrete probe laser wavelengths (above the photodetachment threshold the dependence on fluence is linear, below it is quadratic). An even/odd alternation is found in the negative ion distribution with gallium arsenide clusters with an odd number of atoms having higher EA’s than their even neighbors. This suggests that the surfaces of the even clusters are extensively restructured in a way which eliminates dangling chemical bonds. For GaxAsy with x+y constant, EA increases with increasing ratio of y to x. The EA of silicon increases smoothly with cluster size extrapolating towards the EA of bulk silicon. Photofragmentation studies show that, like the corresponding positive ions, silicon and germanium negative ions wit...

Semiconductor cluster beams: One and two color ionization studies of Six and Gex

Supersonic beams of clusters of Si and Ge atoms have been produced by laser vaporization followed by supersonic expansion in a helium carrier. The cluster beams were characterized by F2(7.9 eV) and ArF(6.4 eV) excimer laser ionization accompanied by time-of-flight mass analysis. In addition, the feasibility of a resonant two-photon ionization (R2PI) spectroscopic study was explored by two-color experiments involving initial excitation with the second (2.36 eV) and third (3.54 eV) harmonics of the Nd:YAG followed by excimer laser ionization. All two-photon ionization processes were found to produce extensive fragmentation of the larger clusters. The observed fragmentation pattern for the silicon and germanium clusters were remarkably similar to each other, but drastically different from that seen for metal clusters in the same apparatus. Unlike metal clusters, which tend to lose one atom at a time, these semiconductor clusters appear to fragment by a fission process, the daughter ions falling almost exclusively in the size range from 6 to 11 atoms. Time delay studies in the two-color experiments established that clusters of both Si and Ge have excited electronic states with lifetimes of approximately 100 ns. This again is dramatically different from the behavior found with metal clusters, and indicates the feasibility of R2PI spectroscopy on these cold semiconductor particles. The existence of such long-lived excited states indicates that there is probably an energy gap between the band of electronic states being excited and the ground electronic state.

Mid-infrared quantum cascade laser based off-axis integrated cavity output spectroscopy for biogenic nitric oxide detection

Tunable-laser absorption spectroscopy in the mid-IR spectral region is a sensitive analytical technique for trace-gas quantification. The detection of nitric oxide (NO) in exhaled breath is of particular interest in the diagnosis of lower-airway inflammation associated with a number of lung diseases and illnesses. A gas analyzer based on a continuous-wave mid-IR quantum cascade laser operating at approximately 5.2 microm and on off-axis integrated cavity output spectroscopy (ICOS) has been developed to measure NO concentrations in human breath. A compact sample cell, 5.3 cm in length and with a volume of < 80 cm3, that is suitable for on-line and off-line measurements during a single breath cycle, has been designed and tested. A noise-equivalent (signal-to-noise ratio of 1) sensitivity of 10 parts in 10(9) by volume (ppbv) of NO was achieved. The combination of ICOS with wavelength modulation resulted in a 2-ppbv noise-equivalent sensitivity. The total data acquisition and averaging time was 15 s in both cases. The feasibility of detecting NO in expired human breath as a potential noninvasive medical diagnostic tool is discussed.

Photodissociation of semiconductor positive cluster ions

Laser photofragmentation of Si, Ge, and GaAs positive cluster ions prepared by laser vaporization and supersonic beam expansion has been investigated using tandem time‐of‐flight mass spectrometry. Si clusters up to size 80, Ge clusters to size 40, and GaAs clusters up to a total of 31 atoms were studied. Si+n and Ge+n for n=12–26 give daughter ions of about half their original size. For both Si and Ge, this apparent positive ion fissioning appears to go over with increasing n to neutral loss of seven and ten, but for Si+n the range of n values where this is observed is rather small. At low fluences, the larger Ge+n clusters up to the maximum size observed (50) sequentially lose Ge10 (and in some cases with lower intensity Ge7). Larger Si+n clusters (n>30) always fragment primarily to produce positive ion clusters in the 6–11 size range with a subsidiary channel of loss of a single Si atom. At high laser fluences, Ge+n also fragments to produce primarily positive ion clusters in the 6–11 size range with an...

Sensitivity enhancement of laser absorption spectroscopy by magnetic rotation effect

The use of magnetic rotation spectroscopy (Faraday effect) to reduce the effects of source noise and improve sensitivity of spectroscopy with color center lasers has been analyzed theoretically and tested experimentally on the vibrational overtone band of NO. Sensitivity improvement of a factor of 30 compared with simple Zeeman modulation has been demonstrated. As an example of this technique, the first observation of the fundamental vibrational band of the OH radical in absorption is reported.

Methane concentration and isotopic composition measurements with a mid-infrared quantum-cascade laser

A quantum-cascade laser operating at a wavelength of 8.1 micrometers was used for high-sensitivity absorption spectroscopy of methane (CH4). The laser frequency was continuously scanned with current over more than 3 cm-1, and absorption spectra of the CH4 nu 4 P branch were recorded. The measured laser linewidth was 50 MHz. A CH4 concentration of 15.6 parts in 10(6) ( ppm) in 50 Torr of air was measured in a 43-cm path length with +/- 0.5-ppm accuracy when the signal was averaged over 400 scans. The minimum detectable absorption in such direct absorption measurements is estimated to be 1.1 x 10(-4). The content of 13CH4 and CH3D species in a CH4 sample was determined.