Supersonic Beam Techniques Research Articles

Molecules in the cold environment of a supersonic free-jet beam: from spectroscopy of neutral–neutral interactions to a test of Bell's inequality

The supersonic free-jet expansion technique has been used in different fields of research in physics, physical chemistry and chemistry to study vibrational and rotational molecular structures in ground and excited electronic energy states as well as in cold chemistry to study chemical reactions in a unique environment. The supersonic beam technique, as a widely used method in laser spectroscopy of molecules, exploits a source of monokinetic, rotationally and vibrationally cold molecules, that are very weakly bound in their ground electronic states (van der Waals molecules). In experiments at Jagiellonian University the supersonic free-jet beam serves as a source of ground-state van der Waals objects in studies of neutral–neutral interactions between group 12 metal (M = Zn, Cd, Hg) and noble gas (NG) atoms. Recently, the method has been applied as a source of entangled 199Hg atom pairs in order to test Bell's inequality in an experiment at Texas A&M University.

Spectroscopic Studies of the S0−S1 Transition of Substituted 1-Aminonaphthalenes in a Supersonic Jet

The spectroscopic properties of a series of 1-aminonaphthalenes were examined in the isolated gas phase by means of laser-induced fluorescence combined with supersonic beam techniques. The aminonaphthalenes differ with respect to the size of the amino group, 1AN (NH2), 1MAN (NHCH3), 1DMAN (N(CH3)2), or the presence of a second substituent in the 4-position: 4MDMAN (CH3) and 4ClDMAN (Cl). Whereas the fluorescence excitation spectra of 1AN and 1MAN show an intense 0−0 transition and a well-resolved vibrational pattern, that of the N,N-dimethylamino derivative 1DMAN displays a particularly complex structure, with a very weak origin and a sudden collapse of intensity at excitation energies above 1000 cm-1. A long low-frequency vibrational progression, which has been assigned to the torsional mode of the (N(CH3)2) around the C−N bond, clearly appears in the spectra of the 4-substituted derivatives. DFT calculations of the ground-state structure of 1DMAN and 4ClDMAN show that the dimethylamino group is twisted by 49.7° relative to the aromatic plane. An analysis of the Franck−Condon distribution along the torsional progression for 4ClDMAN indicates a decrease of the dimethylamino twist angle in S1 by about 20°. The energy threshold for the drop in fluorescence intensity, depending on the nature of the substituent in the 4-position (1200 cm-1 for 4MDMAN, 700 cm-1 for 4ClDMAN), indicates the presence of an efficient internal conversion process, similar to what has previously been observed in solution. Complexation of 1DMAN with acetonitrile reduces the internal conversion efficiency, as can be deduced from the enhanced fluorescence intensity as well as from the lengthening of the fluorescence decay time. The collapse of the FES spectrum of the 1DMAN/CH3CN complex (1:1) occurs at a significantly higher excess energy (1500 cm-1) than in the case of bare 1DMAN.

Pulsed field ionization-photoion spectroscopy using two-bunch synchrotron radiation: Time-of-flight selection scheme

We have demonstrated that the time-of-flight (TOF) selection method for pulsed field ionization (PFI) photoelectron detection [Jarvis et al., Rev. Sci. Instrum. 70, 2615 (1999)] can also be applied for the detection of PFI-photoions (PFI-PIs) using the two-bunch synchrotron radiation at the Advanced Light Source. By employing the supersonic beam technique to lower the translational temperature of the sample gas, we show that background prompt ions formed in direct and spontaneous autoionization processes arrive at the ion detector in a pattern similar to that of the vacuum ultraviolet light bunches. The PFI-PIs formed at dark gaps can be designed to arrive at the detector in between adjacent prompt ion peaks, enabling the gating of the PFI-PI signal with only minor contamination from background prompt ions. This experiment has revealed important considerations for the design of a general TOF selection scheme for PFI-PI detection using synchrotron radiation.

Enhancement of surface decomposition using supersonic beam: direct evidence from GaN quantum dot formations on AlGaN surfaces in gas-source molecular beam epitaxy

The enhancement effect of surface decomposition using supersonic beam was found for the first time, in the fabrication of GaN quantum dots on AlGaN/6H-SiC(0 0 0 1) surfaces by gas-source molecular beam epitaxy. GaN quantum dots were successfully fabricated using Si which was supplied by supersonic beam of CH 3SiH 3, while the GaN dots could not be formed using usual beam of CH 3SiH 3. The optical properties of the fabricated GaN quantum dots were investigated by photoluminescence measurements. The advantage by using supersonic beam technique in the enhancement of surface reaction was demonstrated.

Photoemission from Ag-clusters in a C 60 matrix

Ag-clusters with a narrow size distribution around 2.8 ± 0.5 nm in diameter have been produced by a supersonic-beam technique and embedded into a matrix of solid C 60. Photoelectron spectra of the valence-band region do not confirm a large charge transfer between the Ag clusters and the C 60 matrix. No plasmon feature is detected in the spectra of the Ag-3 d core level of the Ag-clusters.

Laser-induced reactions of semiconductor surfaces

Laser-induced reactions of Ge(111), Si(111), GaAs(100), and InP(100) surfaces with chlorine under 355-, 560-, and 1064-nm laser irradiation have been investigated using a supersonic beam technique and time-resolved mass spectrometry. It has been found that the reaction yields depend not only on the laser fluence and wavelength, but also on the translational energy of the incident chlorine molecules. A possible mechanism of laser-induced reactions is proposed.

Vibrational cooling of aniline molecules by laser evaporation from a cryogenic matrix

The resonance-enhanced two-photon ionization spectra of aniline molecules evaporated from a carbon dioxide matrix by laser irradiation have been measured. An intensity decrease of “hot” bands in the spectrum of the 1B2 ← 1A1 electronic transition of aniline molecules evaporated from the matrix was detected. We assume that this effect is connected with the supersonic expansion of dense matrix vapour. Although the vibrational temperature (170±30 K) attained is quite high, it is believed that this method might be very promising as a supersonic beam technique and for introducing complex thermally labile compounds into a supersonic free jet.

Energy transfer from rare gases to surfaces: Collisions with gold and platinum in the range 1-4000 eV.

We have measured the energy transferred to a gold surface by impinging ${\mathrm{He}}^{+}$, ${\mathrm{Ar}}^{+}$, and ${\mathrm{Xe}}^{+}$ ions with kinetic energies in the range 5\char21{}4000 eV. This same quantity has been determined for He, Ar, and Xe atoms colliding with a Pt(111) surface. The ion studies employed a novel highly sensitive pyroelectric calorimeter together with a carefully designed compact ion gun. Pulses of nearly monoenergetic ions from the gun were directed at a gold film evaporated directly onto a pyroelectric material that develops a voltage proportional to the energy deposited. The atomic studies were made with supersonic beam techniques, whereby energy transfer is inferred from time-of-flight distributions of the incident and scattered species. The results from these very different experiments are in good agreement and give a fairly complete picture of energy transfer from incident rare-gas atoms and ions to these heavy-metal surfaces. For energies above about 10 eV, the ions transfer at least 60% of their energy, with Xe transferring the most and He the least. For lower incident energies, the energy transfer decreases, approaching zero-energy intercepts of \ensuremath{\sim}60%, 20%, and 5% for Xe, Ar, and He, respectively. The implications of these experimental results for the effective-mass concept, the binary-collision model, low-energy stopping powers, lattice penetration, and the theory of physical sputtering are considered, and we address the relevance of these findings to the technologically important processes of plasma etching and deposition of sputtered thin films and to particle-spacecraft interactions and controlled thermonuclear fusion.

Atomic versus molecular reactivity at the gas-solid interface: The adsorption and reaction of atomic oxygen on the Si(100) surface.

The reaction of atomic oxygen with the Si(100) surface has been examined by employing supersonic beam techniques. Atomic oxygen adsorbs with unit probability on the clean Si(100) surface. The rate of oxidation decreases rapidly with increasing coverage up to \ensuremath{\sim}3--4 monolayers, followed by a regime that exhibits a weaker dependence on coverage. At surface temperatures above \ensuremath{\sim}1000 K, atomic oxygen reacts with the substrate to produce SiO(g). The kinetics of this reaction depends on the nature of the gas-phase reactant: A single stable surface intermediate is formed from O(g), whereas two intermediates are implicated from ${\mathrm{O}}_{2}$(g).

Spectroscopic predictions for alkali-atom-alkali-halide reaction intermediates

The semi-empirical Roach-Child model is used to predict ionisation potentials, rotational constants, vibrational frequencies and the first three electronic absorption bands for the reaction intermediates Na2Cl, K2Cl and NaKCl, which are known to be isolatable by supersonic beam techniques.

Reactive atom–surface scattering: The adsorption and reaction of atomic oxygen on the Si(100) surface

The adsorption and reaction of atomic oxygen on the Si(100) surface has been examined by employing supersonic beam techniques, x-ray photoelectron spectroscopy, and mass spectrometry. Atomic oxygen adsorbs with a unit probability of adsorption on the clean Si(100) surface, independent of incident translational energy, incident angle, and surface temperature. The probability of adsorption decreases monotonically with increasing coverage. At surface temperatures above ∼1000 K, the adsorption of atomic oxygen results in the gasification of the substrate, producing SiO(g). In comparison to molecular oxygen for this reaction, where two surface intermediates were implicated, only one surface intermediate is formed from the reaction of atomic oxygen with the Si surface. We suggest this is due to the ability of atomic oxygen to attach to the dangling bond states present on the Si(100) surface.

Reactive Atom-Surface Scattering the Adsorption and Reaction of Atomic Oxygen on the Si(100) Surface

ABSTRACTThe adsorption and reaction of atomic oxygen on the Si(100) surface has been examined by employing supersonic beam techniques, X-ray photoelectron spectroscopy and mass spectrometry. Atomic oxygen adsorbs with a unit probability of adsorption on the clean Si(100) surface. The probability of adsorption decreases monotonically with increasing coverage. At surface temperatures above approximately 1000 K. the adsorption of atomic oxygen results in the gasification of the substrate, producing SiO(g). In comparison to molecular oxygen for this reaction, where two surface intermediates were implicated, only one surface intermediate is formed from the reaction of atomic oxygen with the Si surface.

Electronic absorption spectra of biphenyl as studied by multiphoton ionization–supersonic beam technique

The vibrational structure of biphenyl is studied by observing the multiphoton ionization spectra due to radiation from a dye (coumarin 540) laser.(AIP)

Linewidths and high resolution structure of the benzene 1B2u←1A1g two photon transition by multiphoton ionization–supersonic beam techniques

The two photon spectra of the 1B2u state of benzene-h6 and benzene-d6 have been examined in a supersonic molecular beam/multiphoton ionization experiment. The effect of channel three nonradiative decay on the resonance linewidths was followed through energies greater than 6000 cm−1 above the origin, 3000 cm−1 past the fluorescence cutoff region. The linewidth data suggest a leveling off of the channel three broadening at energies greater than T00+6000 cm−1. A new progression in the benzene-h6 spectrum was revealed adjacent to the 1410 1n0 progression (n=1 to 5) which had previously been obscured in the rotational envelope of these bands. New structure and well resolved expected structure is seen in the benzene-d6 spectrum as the 1410 1n0 progression is followed out to n=5.