W. Ronald Gentry

Ten-microsecond pulsed molecular beam source and a fast ionization detector.

We describe a pulsed gas valve which we have developed for use as a molecular beam source. In order to observe the performance of the pulsed beam source, we also have developed an ionization detector with a rise time of about 1 micros. The pulsed valve produces very intense supersonic molecular beam pulses of about 10 micros duration for light gases such as H2 and He, and of somewhat longer duration for heavier gases. As a new tool for the study of molecular collisions, the pulsed beam technique offers substantial advantages over the conventional continuous-beam method for experiments which are limited by the signal-to background ratio for scattered products.

Direct measurement of the size of the helium dimer

The relative transmissions of helium dimer and helium atom beams through a set of nanoscale sieves were measured as a function of hole size in the range from 98–410 nm. From the relative transmission coefficients, the mean internuclear distance of helium dimer was determined to be 〈r〉=62±10 A. This enormous bond length—by far the largest known—confirms recent theoretical estimates of the potential energy well depth e and the extremely small binding energy Eb of helium dimer. The ranges of these parameters corresponding to the experimental uncertainty in 〈r〉 are e/k=10.88–10.98 K and Eb/k=0.65–1.30 mK, where k is the Boltzmann constant.

The dynamics of ethylene dimer infrared photodissociation in pulsed molecular beamsa)

Molecular beam experiments have been carried out on the infrared photodissociation of the (C2H4)2, (C2D4)2, and (C2H4)⋅ (C2D4) van der Waals molecules. Measurements of the frequency and fluence dependences of the photodissociation cross sections were performed over the CO2 laser frequency range from 900 to 1100 cm−1, which covers the region of the ν7 out‐of‐plane vibration of C2H4 and the ν12 in‐plane vibration of C2D4. In addition, the monomer product speed and angle distributions were measured in order to obtain information on the final energy disposition in the products. Absorption in either vibrational mode induces dissociation of the cluster, but the linewidth associated with excitation of the ν7 mode of C2H4 is about four times larger than that associated with excitation of the ν12 mode of C2D4. Two‐laser ‘‘hole‐burning’’ experiments demonstrate the homogeneity of the ground‐state populations. The absorption intensities are not simply proportional to the number of molecules in the cluster capable of absorbing at a given frequency but instead appear to contain information on the structure of the dimer. The dissociation products are scattered isotropically in the center of mass system, with average recoil velocities which correspond to only a small fraction of the available energy appearing in translation. In the case of (C2H4)2, the lack of vibrational modes at the corresponding product internal energies implies that the remaining energy goes into product rotation. Based on the rather detailed spectroscopic and dynamical information provided by these experiments, and the assumption of a skewed parallel structure for the dimer, qualitative arguments are presented that the observed linewidths are determined by the rates of the mode‐specific anharmonic coupling of the monomer vibrations to the van der Waals coordinates, but not necessarily by the vibrational predissociation rates.Molecular beam experiments have been carried out on the infrared photodissociation of the (C2H4)2, (C2D4)2, and (C2H4)⋅ (C2D4) van der Waals molecules. Measurements of the frequency and fluence dependences of the photodissociation cross sections were performed over the CO2 laser frequency range from 900 to 1100 cm−1, which covers the region of the ν7 out‐of‐plane vibration of C2H4 and the ν12 in‐plane vibration of C2D4. In addition, the monomer product speed and angle distributions were measured in order to obtain information on the final energy disposition in the products. Absorption in either vibrational mode induces dissociation of the cluster, but the linewidth associated with excitation of the ν7 mode of C2H4 is about four times larger than that associated with excitation of the ν12 mode of C2D4. Two‐laser ‘‘hole‐burning’’ experiments demonstrate the homogeneity of the ground‐state populations. The absorption intensities are not simply proportional to the number of molecules in the cluster capable of...

Resolved single‐quantum rotational excitation in HD+He collisions: First results from a unique pulsed molecular beam apparatus

Differential cross sections are measured for a well−resolved single−quantum rotational transition for th sampel system HD(J=O)+He→HD(J′=0,1)+He.(AIP)

State‐resolved differential cross sections for the reaction D+H2→HD+H

Differential cross sections for the reaction D+H2→HD+H were measured with a novel crossed molecular beam technique, in which a pulse of energetic D atoms is formed by 193 nm photolysis of D2S and crossed with a pulse of H2 at a variable intersection angle. Speed distributions of the product HD, measured by time‐of‐flight to a mass spectrometer detector, show clearly resolved peaks for v=0 and v=1 at center‐of‐mass scattering angles near 180°, for an initial relative kinetic energy of 0.95 eV. Analysis of the data by computer simulation of the experiment yields a v=0/v=1 population ratio of 5.1, and average rotational energies of 0.26 eV in V=0 and 0.11 eV in v=1.

Sub-miniature ExB sector-field mass spectrometer☆

A novel sub-miniature double-focusing sector-field mass spectrometer has been fabricated at the University of Minnesota using a combination of conventional machining methods and thin film patterning techniques typically used in the sensor technology industry. Its design is based on the mass separation capabilities of a 90° cylindrical crossed electric and magnetic sector-field analyzer with a 2-cm radius, which under proper conditions is able to effectively cancel the angular and chromatic dispersion of the ion beam, thus improving the resolving power of the instrument. Simulations using finite element analysis and computer modeling were employed to verify and optimize the performance of the proposed instrument before and during its fabrication. The prototype was able to attain a resolving power of 106 full-width at half-maximum (FWHM), a detection limit close to 10 parts per million, a dynamic range of 5 orders of magnitude and a mass range up to 103 Da. Its overall size, including the magnet assembly, is 3.5 cm wide, 6 cm long and 7.5 cm tall, it weighs 0.8 kg, and its power consumption was measured to be 2.5 W. The performance of the instrument was found to be comparable to that of commercial residual gas analyzers, at a fraction of the cost. All these characteristics make this miniature mass spectrometer suitable for portable and low-cost analytical instrumentation.

Long‐range interactions of ions with atoms having partially filled p subshells

We derive analytic Born–Oppenheimer potential energy functions, including the charge–quadrupole and charge–induced dipole contributions, for the long‐range interaction of a positive or negative ion with an atom having a partially filled p subshell. The charge–quadrupole term vanishes at large separations for the 2 P 1/2 states of p 1 and p 5, the 3 P 0 state of p 2 and p 4, and all states of p 3. For 2 P and 3 P states the potentials at very large separations are characterized by M J and at smaller separations by M L . The switchover is controlled by the ratio of the spin–orbit energy to the total anisotropy of the interaction potential. Plots of the positive and negative ion interactions with C(3 P), O(3 P), and F(2 P) are presented as illustrations.

Quantum vibrational transition probabilities from real classical trajectories: Collinear atom--diatom collisions

Our previous theory of vibrationally inelastic atom–diatom collisions, which is based on an approximate correspondence between the classical and quantal equations of motion, is extended here to the problem of translational–vibrational and vibrational–vibrational energy transfer in collisions of two identical diatoms. Parameters describing the quantal motion of harmonic oscillators subject to an interaction potential which includes only terms linear and bilinear in the two oscillator coordinates are evaluated from exact classical trajectories for the full potential. The formalism makes it possible to distinguish between the vibrational–vibrational and translational–vibrational energy transfer modes and to isolate the intermode coupling terms. The validity of approximations based on a reduced state basis is easily evaluated. The model results are in generally excellent agreement with exact fully quantal calculations, and they reproduce well the dependence of the transition probabilities for collinear H2+H2 ...

State‐to‐state vibrational excitation of I2 in collisions with He

Cross sections for v=0→1, 2, 3 excitation in I2+He collisions have been measured over the kinetic energy range from threshold to 0.4 eV by means of a new experimental technique in which pulsed molecular beams are crossed at a variable intersection angle and scattered products are state selectively detected by laser induced fluorescence. The values of the measured cross sections range from about 0.2 A2 for v=1 at 0.4 eV to about 10−5 A2 for v=3 at the low‐energy detection limit. The kinetic energy dependences of the cross sections for exciting v=1, 2, and 3 are approximately linear, quadratic, and cubic, respectively. The experimental cross sections are compared with those predicted by a simple classical model and by the detailed quantum calculations reported by Schwenke and Truhlar in the accompanying article. It appears that vibrational excitation in this system comes primarily from nearly impulsive collisions with the steeply repulsive part of the interaction potential in approximately collinear geometr...

Rotational excitation in the small‐angle scattering of protons from diatomic molecules

Data for the small‐angle scattering of protons from several polar and nonpolar molecules in the energy range 10 to 30 eV indicate that the presence of a dipole moment in the neutral molecule greatly enhances the rotational excitation process in ion‐molecule collisions. Within our experimental energy resolution, no rotational excitation was observable for proton collisions with H2, HD, D2, or N2. There is, however, some evidence for a small amount of rotational excitation of the dipolar molecule CO, and for HCl and HF large amounts of rotational excitation were observed. The inelasticity was found to be greater for HF than for HCl, consistent with the relative dipole moments and moments of inertia of the two molecules. The average excitation energies for HF measured at fixed scattering angle are qualitatively in agreement with a classical perturbation calculation for pure rotational excitation. For HCl, the classical perturbation calculation gives a rotational excitation energy much lower than the average ...