James J. Wynne

Boron clusters (Bn, n = 2–52) produced by laser ablation of hexagonal boron nitride

Abstract We report the discovery of boron clusters (B n , n = 2–52) formed by ablation of hexagonal boron nitride with a 532 nm laser, creating species which are ionized with a 193 nm UV laser and analyzed with a time-of-flight (TOF) mass spectrometer.

Frequency doubling in KB5O8⋅4H2O and NH4B5O8⋅4H2O to 217.3 nm

We have produced tunable phase‐matched second‐harmonic radiation in a KB5O8⋅4H2O crystal between 217.3 and 234.5 nm. Phase matching was achieved by crystal rotation. We also studied frequency doubling in NH4B5O8⋅4H2O.

Tunable coherent ir source based upon four‐wave parametric conversion in alkali metal vapors

A new type of near‐ir source producing a coherent output with a spectrally narrow (∼ 0.1 cm−1) and continuously tunable frequency is described. It utilizes a four‐wave mixing process in alkali metal vapor with input beams provided by two nitrogen‐laser‐pumped dye lasers. Tunable output has so far been observed in the range 2–5.4 μ.

Bound, odd-parity J = 1 spectra of the alkaline earths: Ca, Sr, and Ba

We have used multiphoton excitation via selected intermediate states to observe and identify triplet Rydberg series of the type msnp3P0 up to high n in Ca, Sr, and Ba. Previous efforts have failed to identify these series beyond n = 7. We present details of our experimental method, give tables of the newly determined energy levels, and describe their analysis by multi-channel quantum defect theory (MQDT). For the most part, the msnp3P0 series in Ca and Sr can be described by a nearly constant quantum defect δ with δCa = 19.7 and δSr = 2.90. The 6s n p3P0 series in Ba is more heavily affected by the configuration interaction, but to a reasonable approximation has δBa ≈ 3.81. For all three elements, we find that the msnp¹P10 series are much more strongly perturbed than the ³P10 series. Most of this paper treats Ba. In addition to the 3P0 states, 3F2 and 1G4 states are measured and tabulated. Further results in Ba include new and/or revised values for some ¹P10 states a revised ionization limit, evidence for an energy-dependent configuration interaction, and an MQDT prediction of the photoabsorption cross section at the ionization limit. This prediction agrees with the most recent experimental cross section. The MQDT analysis of Ba is described in great detail to make it useful as a guide to the reader who wishes to become a user of MQDT.

Rydberg–Rydberg transitions of NO using an optical–optical double resonance multiphoton ionization technique

New Rydberg–Rydberg transitions of 14N 16O have been observed by the technique of optical–optical double resonance multiphoton ionization (OODR‐MPI). The analysis of S 2Σ+–A 2Σ+ (0,0), (1,1) and 4f–A 2Σ+ (1,1) bands are reported. Molecular constants of the S 2Σ+ v=0,1 levels are in good agreement with those derived from absorption data. The structure of the 4f(v=1) level of the 14N 16O isotope can be fully investigated for the first time since the OODR‐MPI spectrum is free from any overlap, in contrast to the corresponding absorption spectral region. No detectable spectral perturbation was observed for levels up to N=10, although the non‐Rydberg G 2Σ−(v=8) level lies at the same energy as the 4f(v=1) level. The 4f(v=1) rotational levels are described by an intermediate case (b)/case (d) coupling scheme and present a similar pattern to that of the 4f(v=0) rotational levels previously studied from absorption data. Rotational constants B1=1.968 cm−1 and αe=0.020 cm−1 have been derived from our analysis and t...

RESONANTLY ENHANCED, NONLINEAR GENERATION OF TUNABLE, COHERENT, VACUUM ULTRAVIOLET (VUV) LIGHT IN ATOMIC VAPORS*

As is well known, atomic vapors, by virtue of their centrosymmetry, do not display quadratic optical nonlinearities. Thus, for example, second harmonic generation is ruled out in these media, provided there are no externally applied fields that destroy the symmetry. Third harmonic generation (THG) in vapors is allowed on the basis of symmetry. We briefly note that the earliest experiments were carried out with noble gas atomic vapors, and that the experiments themselves consisted in measuring the so-called hyperpolarizabilities.s2 These nonlinearities were weak compared t o those in crystalline solids by virtue of the much lower densities of the vapors and the nonresonant nature of the nonlinearities. The lowest lying excited states of noble gas atoms were far above the photon energies involved in these third harmonic generation experiments. The r e ~ e a r c h e r s ~ ~ a t Stanford University who pioneered the use of THG in gases as a practical source of coherent ultraviolet light studied atomic systems other than the noble gases-specifically, the alkali metals-using fixed-frequency input beams. They obtained for the first time practically useful third harmonic conversion efficiencies and showed generally that the large nonlinearities were due t o resonance enhancements. The advantage gained in using metal vapors stems from the fact of their generally low first ionization potentials. Consequently, various bound excited states lie in an energy range where resonance enhancement is possible with the visible input, or VUV third harmonic output beams. The work of Harris et al .3-5 stimulated the interest of various other groups,6-8 each of which then independently focused on the further advantages t o be gained in attaining exact two-photon resonance. These advantages are based upon the lack of linear absorption or dispersion associated with a twophoton resonance. Our own approach, which is characterized experimentally by the use of nitrogen-laser-pumped dye lasers t o provide the input beams, is perhaps best suited t o take advantage of two-photon and other resonance enhancements. It also enables the VUV output t o be tuned. Technical aspects of the dye lasers we employ are adequately described in Reference 9. In the present paper the emphasis will be on physical factors determining the relative VUV output in the various tuning ranges. Before actually discussing VUV generation, however, we shall describe at some length a multiphoton ionization experiment that graphically illustrates the importance of two-photon resonance in nonlinear phenomena in gases and also serves t o acquaint the reader with the arrange-

Generation of the rotationally symmetric TE 01 and TM 01 modes from a wavelength-tunable laser

A nitrogen-laser-pumped dye laser has been made to operate in the rotationally symmetric TE01 or TM01 modes. The modes are preserved as the laser wavelength is varied. The mode selector allows convenient switching from one mode to the other. The theory of the operation of the mode selector is described.

Photoionization and photofragmentation of BxNy clusters produced by laser vaporization of boron nitride

Vaporization of hexagonal boron nitride with a 532 nm laser, followed by supersonic expansion cooling, produces a variety of BxNy clusters which are detected by photoionization of the neutral clusters with a 194 nm laser, followed by time‐of‐flight (TOF) mass spectrometry. At low 194 nm photoionization fluence (<50 mJ/cm2), the mass spectrum shows resolved peaks corresponding to the clusters By+1N+y for y=1–8, as well as an unresolved region corresponding to clusters ranging from atomic mass unit 100–1000. At higher photoionization fluence, resolved peaks, corresponding to B+x clusters for x=2–100, appear and grow with increasing fluence. Concommitantly, the envelope of the unresolved region changes shape and grows with a diminished dependence on fluence, indicating that the unresolved clusters are being photofragmented as well as photoionized by the 194 nm radiation. The products of such photofragmentation are the aforementioned B+x clusters. This process is modeled using an Arrhenius relation to describ...

Passive Q switching of a CO 2 laser near 9.2 &#181; using CF 2 Cl 2

CF 2 Cl 2 is found to cause passive Q switching on practically all of the lines in the R branch of the 00\deg1-02\deg0 vibrational-rotational band of CO 2 .

Pre‐College Physics Education Programs from the Research Community

With the 1980s came the recognition that a crisis is developing in American education and in science and mathematics education in particular. Thus one goal coming out of President Bushs 1989 Education Summit is that “US students will be first in the world in science and mathematics” by the year 2000. Here we consider the response of three sectors of the physics research community—the professional scientific societies, the national laboratories (see figure 1) and industry—and survey the programs they offer for pre‐college students and teachers. There are too many such programs for our treatment to be comprehensive and so we have chosen a representative selection. These same organizations also have many projects that deal with curriculum reform and teaching materials, as Gerhard Salinger discusses in his article on page 39. The box on page 50 summarizes the activities open to the individual researcher.