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A panel discussion: Nine specialists foresee an expanded frequency range, new kinds of lasers, and many novel applications.

In the 1960s and 1970s, spectroscopists developed a host of nonlinear techniques to measure the interaction of light and matter with a resolution fine enough to test quantum electrodynamics and optically detect weak interactions in atoms.

Gerhard Herzberg, Nobel Laureate in Chemistry, had broad interests and achieved supreme accomplishments in physics, chemistry and astrophysics. The high points in his research were many. In physics they were the work on atomic hydrogen and helium, and evaluations of energy levels and constants of molecular hydrogen and its isotopes. His determinations of many molecular structures and discoveries of spectra of the free radicals CH 2 and CH 3 , and of the Rydberg molecule H3, were each outstanding contributions in chemistry. In astrophysics, his reproduction in the laboratory of the spectrum of CH + , proving its presence in the interstellar medium, and of spectra of C 3 and H 2 O + and their presence in comets, his observation of the quadrupole spectrum of H 2 and his discovery of hydrogen in the atmospheres of the planets opened up new applications of spectroscopy for our knowledge of the universe. His classic volumes on molecular spectra and molecular structures remain as encyclopaedias of molecular knowledge for all time. And his famous laboratory, the ‘Temple of spectroscopy’, served as a home for hundreds of scientists around the world, and helped to bring Canadian science to international prominence.

Gerhard Herzberg, Nobel Laureate in Chemistry, had broad interests and achieved supreme accomplishments in physics, chemistry and astrophysics. The high points in his research were many. In physics they were the work on atomic hydrogen and helium, and evaluations of energy levels and constants of molecular hydrogen and its isotopes. His determinations of many molecular structures and discoveries of spectra of the free radicals CH 2 and CH 3 , and of the Rydberg molecule H3, were each outstanding contributions in chemistry. In astrophysics, his reproduction in the laboratory of the spectrum of CH + , proving its presence in the interstellar medium, and of spectra of C 3 and H 2 O + and their presence in comets, his observation of the quadrupole spectrum of H 2 and his discovery of hydrogen in the atmospheres of the planets opened up new applications of spectroscopy for our knowledge of the universe. His classic volumes on molecular spectra and molecular structures remain as encyclopaedias of molecular knowledge for all time. And his famous laboratory, the ‘Temple of spectroscopy’, served as a home for hundreds of scientists around the world, and helped to bring Canadian science to international prominence.

The status of our knowledge of the dissociation energy of molecular hydrogen (H2, HD, D2) from the earliest determinations in 1926–1927 is briefly reviewed. Present values obtained by high-resolution laser spectroscopy and by ab initio computations that include nonadiabatic, relativistic, and radiative corrections are in good agreement to 2 x 10–6.PACS No.: 33.15Fn

Sum-frequency generation with electromagnetically induced transparency (EIT) has been examined in a hydrogen discharge. EIT is demonstrated in the $3p\ensuremath{-}1s$ and $4p\ensuremath{-}1s$ transitions at 102.6 and 97.3 nm, respectively, with increased efficiency of generation arising from higher H-atom density, although competing effects due to the presence of ${\mathrm{H}}_{2}$ distort the line shapes of these spectra. Only for shorter-wavelength transitions (such as $7p\ensuremath{-}1s$ and $8p\ensuremath{-}1s)$ that show very large shifts and distortions is there a decrease in intensity in comparison with generated radiation from H atoms external to the discharge.

Sum-frequency generation enhanced by electromagnetically induced transparency (EIT) has been extended to the production of extreme ultraviolet (XUV) radiation in the range 97.3\char21{}92.6 nm from $np\ensuremath{-}1s$ transitions $(n=4\char21{}8)$ in atomic hydrogen. Pulsed radiation was generated by strong coupling of the $\mathrm{np}$ and $2s$ levels with laser radiation at Balmer wavelengths, and simultaneously (weaker) coupling of the $2s$ and $1s$ levels by two-photon resonance with 243-nm laser radiation. Investigations were carried out over a range of laser intensities and products of interaction length and atomic density $(\mathrm{NL}).$ As the product $\mathrm{NL}$ increased, the XUV intensities were greatly enhanced by EIT, and at high $\mathrm{NL}$ values, the spectral distributions of some of the generated signals were found to be distorted by the presence of molecular hydrogen. Calculated profiles, modified to incorporate the effects of nearby molecular resonances, were shown to be in good agreement with the observed spectra.

Emission spectra (in the region 100–200 nm) of the excimers Ar2*, Kr2*, and Xe2* produced in a dc jet discharge with supersonic expansion were used to determine vibrational temperatures of the excimers, and to analyze the process of excimer relaxation from high vibrational levels near dissociation of the A3Σ+u(1u) states to the lowest levels. The observed narrow bandwidths of the second continua yielded population distributions with at least 80% in the v=0 levels at thermal equilibrium, for all three gases, corresponding to vibrational temperatures of 280, 95, and 82 K for Ar2*, Kr2*, and Xe2*, respectively. Changes in relative intensities of the first and second continua with pressure were monitored by optical pumping of atoms in the 1s5 metastable level [precursor of the excimer A3Σ+u(1u) state], and the resulting difference spectra were analyzed in terms of vibrational relaxation to the v∼0 levels. A simple model based on collisional relaxation was developed to calculate the VUV band spectra. Compariso...

Optogalvanic spectra have been obtained arising from transitions between electronically excited states of excimers formed in a dc discharge with supersonic expansion. In this method, tunable cw laser radiation is incident on the discharge while optogalvanic signals are measured directly across the discharge electrodes. The primary advantage of the method is its high sensitivity, as known in atomic spectroscopy. As an example, an optogalvanic spectrum of the 3 Π g ← 3 Σ u + transition of Ar 2 ∗ near 990 nm is shown.

Emission spectra of the rare‐gas excimers Ar2*, Kr2*, and Xe2* were excited in a dc jet discharge with supersonic expansion. Absorption and fluorescence emission measurements provided atomic population densities for levels of the 1s manifold. Changes in intensities of the atomic resonance lines and the VUV bands were examined as the plasma was irradiated with laser radiation tuned to specific atomic transitions between levels of the 1s and 2p manifolds. This technique of optical pumping has established that excimers in the A 3Σ+u state are the main contributors to the observed bands from this source. Rate equations were developed to explain the observed intensity changes. Comparisons of the calculated and observed changes indicated that population mixing amongst levels of the 1s manifold caused by electron collisions is an important process in regulating the population in the 1s5 atomic level that leads to the formation of rare‐gas excimers by collisions with ground level atoms.

Four-wave mixing in atomic hydrogen with the generation of coherent Lyman- beta radiation at 102.6 nm was investigated experimentally and theoretically in the low density limit. We demonstrate that in this doubly-resonant system, the application of intense laser radiation to strongly couple the 2s and 3p levels causes transparency in the 3p from 1s transition, leading to reduced absorption of the generated Lyman- beta radiation, and to increased conversion efficiency.

Harry Lambert Welsh began a long association with the University of Toronto with his enrolment in undergraduate physics in 1926. He brought fame to his Alma Mater with pioneering studies in molecular spectroscopy and intermolecular forces, and he played a major role in the development of the Physics Department with his introduction and establishment of research groups in theoretical, atmospheric, and high-energy physics. Undoubtedly, Harry Welsh’s greatest achievement was the stimulation for scholarly research engendered in 65 Ph.D. students who had the privilege and pleasure to study under his supervision over a period of four decades. These scientists have made, and are continuing to make, important contributions to research in a variety of ways in universities, industry, and government institutions, across Canada and in other countries.

Nonlinear optical generation using electromagnetically induced transparency is demonstrated with atomic hydrogen as a test medium. With the application of a strong-coupling laser field to the 3p-2s transition, the resonant Lyman-β (3p-1s) transition becomes transparent. The generated-sum-frequency Lyman-β radiation at 103 nm can continue to grow without suffering resonant absorption and phase mismatching

Fluorescence-excitation spectra of HD have been obtained in the region of ~84 nm showing two dissociation continua corresponding to the photofragments H (1s) +D (2s) and H (2s) +D (1s). Pulsed and tunable coherent radiation was used for excitation with detection by fluorescence emission of Ly-α radiation. By application of a delayed electric field for quenching of the photofragments, only the 2s atoms were selected for detection. At the high resolution achieved, the profiles of the two dissociation limits were clearly observed, and values of the thresholds obtained. The two limits are separated by 21.61±0.10 cm-1, almost 1 cm1 less than the expected difference in excitation energies of the D(2s) and H(2s) atoms. The lower limit, with subtraction of the H(2s-1s) energy, gives a value of D0(HD) = 36 405.83±0.10 cm-1 for the dissociation energy of the ground electronic state of HD, which agrees with the recent ab initio calculation1 of 36 405.784+0.10 cm-1. The apparent discrepancy (almost 10 times the quoted error of measurement) of the value derived from the higher limit remains to be explained. When the present value of D0(HD) is combined with known values of the ionization potentials of HD and H, the value Do(HD+) = 21 516±0.10 cm-1 is obtained for the dissociation energy of the ion, again in good agreement with theory.

Emission spectra of the Ar/sub 2/ band at 128 nm, excited in a pulsed discharge with supersonic jet expansion, have been investigated under a range of gas pressures and voltages. Quartz, ceramic, and tungsten nozzles have been used. A significant observation is the narrow bandwidth of 9 nm at approximately 1 atm nozzle pressure, which decreases to 7.2 nm (FWHM) at 6.6 atm. Such a narrow bandwidth demonstrates that under the present experimental conditions, the observed band arises from the lowest vibrational levels of the excimer state. >

Resonantly enhanced second-harmonic generation in atomic hydrogen was investigated near 121.6 nm. With the application of a dc electric field, it is shown that coupling of the 2s and 2p states produces resonantly enhanced nonlinear susceptibility and reduced absorption at the second-harmonic wavelength, resulting in an increased net efficiency of the second-harmonic-generation process.

Laser emission at 126 nm has been obtained from excimers of Ar(2) excited electrically in a nozzle discharge followed by supersonic expansion. The direct measurement of light amplification with a tunable VUV probe leads to a gain of3 cm(-1). This high gain arises from efficient population inversion of low vibrational and rotational levels of the excimer state brought about by the supersonic expansion and cooling of the discharge plasma.

Tunable, coherent radiation has been generated in the region 105 to 87.5 nm by resonant four-wave frequency mixing in Hg vapor. A glass capillary array forms an efficient XUV window with 50% transmission. An absorption spectrum of N(2) was recorded at 89 nm with a resolving power of 2 x 10(5).

Radiative lifetimes of rovibronic levels of the B', (2)Delta state of NO (from u' = 0,. . . ,8) have been obtained along with those of the perturbing levels F(2)Delta(u' = 0,1,2) and N(2)Delta(u' = 0). The resolution (~0.1 cm(-1)) and intensity of the coherent tunable VUV source permitted excitation of individual rotational levels and measurement of corresponding fluorescence intensity. Lifetimes for u' = 0, . . . ,8 levels of the B' (2)Delta state were found to be in the range tau(0)~ 220 +/- 20 nsec, which is approximately twice the values reported for electron-excitation experiments.

Coherent second-harmonic radiation has been generated in atomic zinc vapor at 1792.5 and 1601 A from the 5(1)S(0)and 4(1)D(2) states, respectively, by two-photon resonance enhancement. Measurements of the ratio of second-to third-harmonic generation suggest the occurrence of static electric fields of ~5 kV/cm. These fields are caused by ionization of ~2% of the atoms in the focal volume of the incident laser beam.

We used a tunable cw dye laser to observe Doppler-free two-photon transitions to alkali-atom Rydberg levels. Real-time signals were obtained in an ionization cell of appropriate construction. The two-photon transition wavelengths to rubidium s levels were measured up to n = 50 with better than 1 x 10(-7) absolute accuracy and can be represented to within the experimental precision by a simple function of the principal quantum number, Other related transitions and future possibilities are considered.

The effect of varying the relative arrival time of the two laser pulses employed in doubly resonant four-wave sum mixing, enhanced by induced transparency, is studied with the aim of optimizing the efficiency of vacuum-ultraviolet generation. With atomic hydrogen as the nonlinear medium, pulsed radiation with wavelengths of 243 and 656 nm and durations of 8 and 14 ns, respectively, is mixed to generate 103-nm coherent radiation. It is shown that by delaying the arrival time of the ground-state pump beam (243 nm) by 2.5-3.5 ns relative to the arrival of the upper-state coupling beam (656 nm), it is possible to enhance the generated intensity by a factor of 2 or more.

Recently a new program of `First Year Seminar Courses' was initiated in the Faculty of Arts and Science at the University of Toronto to introduce in-coming undergraduates to the excitement of university scholarship. These are full- year elective courses, and in each course students work closely with a faculty member in groups of no more than 20 students. I shall review my experiences in these past three years in organizing a course on light, optics, and photonics. The primary aim was to use the rich story of light to bring together students of a variety of intellectual interests, in the hope of demonstrating the valuable knowledge that the cultures of the humanists and scientists can bring to an understanding of the modern world. As we are all aware, the story of light is so all- embracing that this course included not only the science of light and optics, with lasers and photonics, but light in the open air, astronomy, religion, literature, art, architecture, film, stage, health and medicine, with several of the topics discussed by guest speakers. After a brief review of the story of light from its earliest beginnings, an outline of the course will be given along with the many contributions by the students and evaluation of their efforts.

It was in the early 1960s, immediately after the development of the ruby and the helium-neon lasers that the first observations were made of optical harmonics, two-photon absorption, stimulated Raman and Brillouin scattering, and self-trapping and focusing of laser beams. This was an exciting period for nonlinear optics, and Charles Townes and his students and collaborators were major players in the exploration of this field of research.

Fluorescence-excitation spectroscopy near \ensuremath{\sim}84 nm has been used to measure the threshold for the dissociation of ${\mathrm{H}}_{2}$(B' $^{1}\mathrm{\ensuremath{\Sigma}}_{\mathit{u}}^{+}$)=H(1s)+H(2s) at 118 377.06\ifmmode\pm\else\textpm\fi{}0.04 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ and the corresponding limit for ${\mathrm{D}}_{2}$ at 119 029.72\ifmmode\pm\else\textpm\fi{}0.04 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$. The onset of dissociation was observed clearly, free from overlapping molecular lines, by the use of delayed quenching of 2s atoms produced by the photoexcitation pulse. These measurements have yielded the values ${\mathit{D}}_{0}$(${\mathrm{H}}_{2}$)=36 118.11\ifmmode\pm\else\textpm\fi{}0.08 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ and ${\mathit{D}}_{0}$(${\mathrm{D}}_{2}$)=36 748.38\ifmmode\pm\else\textpm\fi{}0.07 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ for the dissociation energies of the ground electronic states X $^{1}\mathrm{\ensuremath{\Sigma}}_{\mathrm{g}}^{+}$, in agreement with the latest ab initio calculations, which include nonadiabatic, relativistic, and radiative corrections. From the present values and recently determined ionization energies of ${\mathrm{H}}_{2}$ and ${\mathrm{D}}_{2}$, the dissociation energies of the respective ions were found to be ${\mathit{D}}_{0}$(${\mathrm{H}}_{2}^{+}$)=21 379.37\ifmmode\pm\else\textpm\fi{}0.08 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ and ${\mathit{D}}_{0}$(${\mathrm{D}}_{2}^{+}$)=21 711.64\ifmmode\pm\else\textpm\fi{}0.07 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$, also in agreement with recent ab initio calculations.

Nonlinear optical generation using electromagnetically induced transparency is demonstrated for the first time. An atomic hydrogen is used as a test atom. By applying a strong‐coupling laser field to the 3p‐2s transition, the resonant 3p‐1s transition becomes transparent with keeping the resonantly enhanced value on the nonlinear susceptibility. The generated sum‐frequency radiation at 103 nm can grow without suffering from the resonant absorption and phase‐mismatching.

Fluoresence-excitation spectroscopy near 84.5 nm has been used to measure the threshold for the dissociation limit of ${\mathrm{H}}_{2}$(B' $^{1}\mathrm{\ensuremath{\Sigma}}_{\mathit{u}}^{+}$)=H(1s)+H(2s) at 118 377.06\ifmmode\pm\else\textpm\fi{}0.04 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$. This yields a value of ${\mathit{D}}_{0}$(${\mathrm{H}}_{2}$)=36 118.11\ifmmode\pm\else\textpm\fi{}0.08 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ for the dissociation energy of the ground electronic state X${(}^{1}$${\mathrm{\ensuremath{\Sigma}}}_{\mathit{g}}^{+}$) of ${\mathrm{H}}_{2}$, in agreement with the latest theoretical calculation which includes nonadiabatic, relativistic, and radiative corrections. When combined with recent determinations of the ionization energy of ${\mathrm{H}}_{2}$ the present ${\mathit{D}}_{0}$(${\mathrm{H}}_{2}$) leads to the value ${\mathit{D}}_{0}$${\mathrm{H}}_{2}^{+}$)=21 379.36\ifmmode\pm\else\textpm\fi{}0.08 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$, also in good agreement with theoretical values.

We show that dc-electric-field coupling of the 2s and 2p states in atomic hydrogen leads to resonantly enhanced second-order susceptibility with reduced absorption at the second-harmonic wavelength, and exact phase matching at the center of the Stark-split components.

Second-harmonic generation in the absence of an externally appplied electric field was investigated near 121.6 nm in atomic hydrogen. It was shown that three-photon ionization produces a local electric field due to charge separation that in turn induces second-harmonic generation in this otherwise isotropic medium. The magnitude of the charge-separation field was determined from the observed second-harmonic power and was found to be in good agreement with a calculated value based on the measured number of photo-ions, the interaction volume, and the incident laser pulse duration.

Emission spectra of the homonuclear rare-gas excimers have been investigated in the wavelength region of 50 to 200 nm by using continuous discharges with supersonic jet expansion. The well-known dimer bands were readily observed in the emission spectra of He, Ar, Kr, and Xe along with an extremely weak band of Ne. A detailed examination of the Ar(2) band at 128 nm was carried out, and its envelope and bandwidth (8.7 nm) indicate significant vibrational cooling owing to supersonic expansion.

Second-harmonic generation in atomic hydrogen is investigated with and without an applied electric field, by tuning laser radiation at 243.1 nm through the 2 2S1/2−1 2S1/2 two-photon resonance. Even at zero applied field, second-harmonic radiation at 121.6 nm was observed. The measured intensity is shown to arise from a charge-separation field caused by ions produced in three-photon ionization. The magnitude of this field is in good agreement with a calculated value based on the measured ion density.

Individual rotational levels of the B′2Δ state (ν′ = 1 – 8) of nitric oxide have been excited using tunable, coherent, vacuum ultraviolet radiation (in the range 140–170 nm) generated by four-wave mixing in magnesium vapour. Excitation spectra have also been obtained for the F2Δ, N2Δ, D2Σ+, E2Σ+, and M2Σ+ Rydbcrg states. Time-resolved fluorescence intensity measurements of rotational levels in ν′ = 1, 2, 3, 5, and 8 of the B′2Δ state indicated single-exponential decay over a large pressure range, resulting in average lifetimes of ~225 ns. Rotational levels in ν′ = 4, 6, and 7 displayed non-single-exponential decay. This behaviour is attributed to enhanced collisional population transfer from the B′2Δ state to the F2Δ and N2Δ states as a result of strong interaction amongst these states.

A fluorescence excitation spectrum of the rare gas dimer XeKr has been recorded at ≈ 123 nm. The analysis of 18 closely-spaced vibronic bands leads to a dissociation energy of 150.5±0.2 cm −1 for the ground state and to estimates of the internuclear distance and dissociation energy of the excited state.

A vernier fringe‐counting circuit designed for use with a Kowalski‐type wavemeter is described. Wavelength measurements made with this device have a high reproducibility of 5×10−9 (or 1/50 of a fringe) and measured accuracy of 4×10−9. The results of accuracy tests are presented along with systematic errors incurred when the wavelengths of two He–Ne lasers (632.9 nm) are compared by the vernier method.

Five single crystals of parahydrogen were grown near the triple point, and their Brillouin spectra examined in known crystal directions. An analysis of the measured frequency shifts of longitudinal and transverse components led to the direct determination of the adiabatic elastic constants at 13.2 K: C11 = 3.34 ± 0.05, C13 = 0.56 ± 0.03, C33 = 4.08 ± 0.06, C44 = 1.04 ± 0.03 kbar. The fifth elastic constant C12 = 1.30 ± 0.05 kbar was evaluated from the relation C11 + C12 = C13 + C33. Values of the following constants were calculated: elastic anisotropy, A = 1.02 ± 0.06, adiabatic bulk modulus, Bs = 1.73 ± 0.04 kbar, and Debye temperature, θD = 111.2 K. All of these values were compared with earlier experimental measurements. The elastic constants were found to be in good agreement with a recent self-consistent phonon calculation by Goldman.

Two-photon Doppler-free spectroscopy has been used to measure Stark shifts of n ${\mathrm{}}^{2}$D Rydberg states of $^{85}\mathrm{Rb}$ (for n=13--55). Scalar and tensor polarizabilities were determined and found to depend on (${n}^{\mathrm{*}}$${)}^{6}$ and (${n}^{\mathrm{*}}$${)}^{7}$, in agreement with theory. The resulting polarizabilities were used to obtain estimates of radial integrals for D-P transitions.

A xenon-chlorine excimer laser was used to irradiate normal endocardium of fresh sheep and pig hearts as well as unfixed human endocardial scar. Forty pulses of 370 J and 35 ns each resulted in penetration of up to 12 mm in normal tissue and only 3.5 mm in scarred endocardium. Dosimetry indicated that the volume of vaporized scarred tissue was 1/10th that of normal endocardium (0.19 to 0.40 versus 1.35 to 3.22 mm 3 /J). Ultrastructurally, there was a sharp demarcation of only 10 μ between the region of injury and normal myocardium, with little evidence of heat injury. The high power and short duration of these lasers coupled with the lack of a boundary zone of injury suggest that excimers may be an ideal tool for arrhythmia ablation.

Vibrationally and isotopically resolved fluorescence excitation spectra of Xe2 at 1300, 1485, and 1500 A have been obtained and analyzed. They correspond to electronic transitions from the van der Waals ground state to the three lowest excited states. Molecules of Xe2 were formed in the v‘=0 level of the ground state using a pulsed supersonic jet, and fluorescence emission was excited by a tunable, coherent, and monochromatic source of vacuum ultraviolet radiation generated by four‐wave‐sum‐mixing in Mg and Zn vapors. For each of the three band systems, unambiguous assignment of upper‐state vibrational quantum numbering was possible, resulting in the determination of accurate upper‐state spectroscopic constants for the first time. From system I (at ∼1500 A) with bands v’=36 to 43, constants for the A1u state of 129,132Xe2 were found to be T’e =63 089.9(6.9), ωe =137.48(34), ωex’e =1.1668(43), and De =4174.4(7.5) cm−1. Band system II (at ∼1485 A) with bands v’=35 to 46, gave constants for the B0+u state: T’e =63 795.6(4.2), ωe =124.85(20), ωex’e =0.9372(24), and De =4446.3(5.2) cm−1. Finally, system III (at ∼1300 A) and bands v’=16 to 27 yielded the following constants for the C0+u state: T’e =75 881.7(1.5), ωe =49.71(10), ωex’e =0.4222(41), and De =1500.0 (3.4) cm−1. Potential curves based on these constants were calculated for all three excited states.

Single crystals of argon were grown and investigated at 82.3 K, near the triple point. X-ray diffraction photographs taken in transmission were used to verify that the crystals were single and to determine their orientation. Brillouin spectra were obtained for several different orientations of three crystals. The observed spectra contained the longitudinal component and at least one, and sometimes both transverse components. Their frequency shifts were used to determine the adiabatic elastic constants at 82.3 K: ${C}_{11}=2.38\ifmmode\pm\else\textpm\fi{}0.04$, ${C}_{12}=1.56\ifmmode\pm\else\textpm\fi{}0.03$, ${C}_{44}=1.12\ifmmode\pm\else\textpm\fi{}0.03$, in units ${10}^{10}$ dyn/${\mathrm{cm}}^{2}$, and the elastic anisotropy $A=2.73\ifmmode\pm\else\textpm\fi{}0.10$. These values differ significantly from earlier results derived from neutron scattering and ultrasonic experiments. However, there is good agreement with recent Monte Carlo calculations which use multiparameter pair potentials and the triple-dipole interaction.

Vibronic spectra of three band systems of 40Ar2 and 36Ar40Ar at 108, 107, and 105 nm were recorded and analyzed. Isotope shifts of bandheads were used to establish vibrational numbering of the observed bands. Rovibronic structure was examined at resolving powers up to 5×105, yielding band centers, rotational constants, and providing evidence of Hund’s case (c) coupling for Ar2. From band system II at 107 nm, with bands involving v’=20–27 and v‘=0–5, improved constants for the ground X 0+g state were obtained: ω‘e=30.68(8), ωex‘e=2.42(5), ωey‘e=−0.062(13), ωez‘e=0.010(2), and D‘e=99.2(10) cm−1, B‘e=0.059 65(8) cm−1, and Re=3.761(3) A. Similarly, constants for v’=20–27 of the B 0+u were evaluated, with estimates of T’e=88 210(400) and De=5640(400) cm−1. Band system I at 108 nm, led to constants for the v’=23–30 levels of the A1u state and to estimates of T’e=87 458(500) and De=5786(500) cm−1. Analysis of band system III at 105 nm yielded new and improved data for levels v’=0–9 of the C 0+u state, including T’e=95 033.6(3), De=465.8(4), ω’e=68.16, ωexe=4.631, ωey’e=0.0479, ωeze=0.0070, and B’e=0.065 24(20) cm−1, the latter resulting in Re=3.596(5) A. Potential energy curves based on these data were calculated for the ground and three excited states.

Nonlinear frequency mixing of laser radiation in Mg, Zn, and Hg vapors has produced coherent radiation, tunable over the range 175–104.5 nm in the vacuum ultraviolet. The resulting radiation is pulsed, monochromatic, and of sufficient intensity for use in high-resolution spectroscopy. Detailed descriptions are given of the dye oscillator–amplifier systems along with the heat-pipe ovens for producing stable densities of the metal vapors. Measurements of intensities, linewidths, and ranges of tunability are presented for each metal vapor.

Measurements of gain and noise in a thermionic detector (with an electrostatically shielded region) using Rb and K vapor are described. An approximate theory for space‐charge limited operation is developed and found to agree with observations. Improvements in diode design are suggested.

Time-resolved fluorescence studies of ${\mathrm{Ar}}_{2}$, ${\mathrm{Kr}}_{2}$, and ${\mathrm{Xe}}_{2}$ formed by supersonic jet expansion and excited by monochromatic, tunable, vacuum-ultraviolet radiation have been used to measure the radiative lifetimes of selected, high vibronic levels of the lowest excimer (${A}_{1u}$) states. For ${\mathrm{Ar}}_{2}$, lifetimes of levels v'=24--30 were found to have an average value of 160\ifmmode\pm\else\textpm\fi{}10 ns; for ${\mathrm{Kr}}_{2}$, levels v'=32--38 had average lifetimes of 56\ifmmode\pm\else\textpm\fi{}5 ns; and for ${\mathrm{Xe}}_{2}$, levels v'=36--43 had average lifetimes of 45\ifmmode\pm\else\textpm\fi{}6 ns. These lifetimes for ${\mathrm{Ar}}_{2}$, ${\mathrm{Kr}}_{2}$, and ${\mathrm{Xe}}_{2}$ differ by factors of approximately 20, 5, and 2, respectively, from values for the v'\ensuremath{\sim}0 levels obtained earlier with high gas pressures and excitation by charged particles or synchrotron radiation. It is shown that these large variations in radiative lifetimes with vibrational level arise from increases in electronic transition moments as internuclear distances increase from equilibrium values to those corresponding to the high vibrational levels. The agreement between all measured lifetimes and calculated values (with spin-orbit coupling included) is well within the accuracy permitted by the presently known spectroscopic constants of ${\mathrm{Ar}}_{2}$, ${\mathrm{Kr}}_{2}$, and ${\mathrm{Xe}}_{2}$.

Depolarized spectra of liquid salol have been observed and analyzed over a range of temperature, from 122 to −22 °C, with the viscosity varying from ∼2 to more than 104 cP. At the lowest temperatures, the spectra exhibit a central line and well‐separated Brillouin components, characteristic of light scattering by oscillatory shear waves. As the temperature is raised, the Brillouin components merge with the central line. At the highest temperatures, broad depolarized doublets with intensity minima at the exciting frequency appear, and are ascribed to light scattering from highly damped shear modes. The spectra observed over the entire temperature range are shown to be in good agreement with the ’’two variable’’ theory of Andersen and Pecora.