F-center Laser Research Articles

The state-to-state predissociation dynamics of OC–HF upon HF stretch excitation

Photofragment angular and state distributions have been measured following the vibrational predissociation of the OC–HF complex. An F-center laser is used to pump the fundamental H–F stretching vibration of the complex and a second F-center laser is used to probe the rotational states of the HF fragment as a function of recoil angle. The complex dissociates via two different sets of channels, one that produces vCO=1, JHF=6,5,4 (intermolecular V–V transfer) and the other vCO=0, JHF=11 (V–R transfer). Analysis of the data gives correlated final state distributions, as well as an accurate value for the dissociation energy (D0) of the complex, namely 732±2 cm−1.

Rotationally inelastic scattering of jet cooled H2O with Ar: State-to-state cross sections and rotational alignment effects

State-to-state rotationally inelastic scattering cross sections of H2O with Ar are measured under single-collision conditions in crossed supersonic jets at a collision energy of 480(90) cm−1. The H2O is initially prepared in its lowest ortho (101) and para (000) rotational levels by supersonic cooling in a Ne expansion, and then excited in the intersection region by single collisions with a second pulsed jet of Ar atoms. Column-integrated densities of H2O in both initial and final scattering states are monitored via direct absorption of narrow bandwidth (Δν≈0.0001 cm−1) infrared light from a continuous wave (cw) F-center laser. Absolute inelastic cross sections for state-to-state collisional energy transfer out of para and ortho initial states are determined from the dependence of infrared absorption signals on collider gas densities. Overall, the results can be approximately characterized by an exponential decrease in cross section with the magnitude of rotational energy transferred, i.e., as suggested by exponential energy gap models. However, at the state-to-state level, a highly structured, nonmonotonic dependence on energy is observed, which indicates a propensity for rotational excitation around the A (in-plane, perpendicular to C2) and C (out-of-plane) principal axes. This preferential state-to-state scattering dynamics reflects an intramolecular alignment of J in the body-fixed frame and is in good qualitative agreement with theoretical classical trajectory predictions. A rigorous comparison is made via full quantum close-coupling scattering calculations on empirical and ab initio Ar-H2O potential energy surfaces, which successfully reproduce all the state-to-state trends observed, but at the more quantitative level appear to overestimate the intramolecular alignment effects experimentally observed.

Quantum state-resolved reactive scattering of F+H2 in supersonic jets: Nascent HF(v,J) rovibrational distributions via IR laser direct absorption methods

Supersonically cooled discharge radical atom sources are combined with high-sensitivity IR absorption methods to investigate state-to-state reactive scattering of F+n-H2→HF(v,J)+H in low-density crossed supersonic jets at center-of-mass collision energies of 2.4(6) kcal/mole. The product HF(v,J) is probed with full vibrational and rotational quantum state selectivity via direct absorption of a single mode (Δν≈0.0001 cm−1), tunable F-center laser in the Δv=1 fundamental manifold with near shot noise limited detection levels of 108 molecules/cm3/quantum state per pulse. The high absorption sensitivity, long mean free path lengths, and low-density conditions in the intersection region permit collision-free HF(v,J) rovibrational product state distributions to be extracted for the first time. Summed over all rotational levels, the HF vibrational branching ratios are 27.0(5)%, 54.2(23)%, 18.8(32)%, and <2(2)%, respectively, into vHF=3:2:1:0. The nascent vibrational distributions are in good agreement with rotationally unresolved crossed-beam studies of Neumark et al. [J. Chem. Phys. 82, 3045 (1985)], as well as with full quantum close-coupled calculations of Castillo and Manolopoulos [J. Chem. Phys. 104, 6531 (1996)] on the lowest adiabatic F+H2 potential surface of Stark and Werner [J. Chem. Phys. 104, 6515 (1996)]. At a finer level of quantum state resolution, the nascent rotational distributions match reasonably well with full quantum theoretical predictions, improving on the level of agreement between theory and experiment from early arrested relaxation studies. Nevertheless, significant discrepancies still exist between the fully quantum state-resolved experiment and theory, especially for the highest energetically allowed rotational levels.

Fabrication of single-mode chalcogenide optical fiber

Long lengths (>150 m) of single-mode chalcogenide optical fiber were fabricated by a double crucible technique. Single-mode transmission through 10 m of continuous fiber was demonstrated using an F-center laser at 2.7 /spl mu/m. The optical loss of this fiber was measured by a standard cutback technique using an FTIR spectrometer and also using an F-center laser, A minimum loss of less than 1 dB/m was obtained.

Near-threshold infrared photodetachment of Al-: A determination of the electron affinity of aluminum and the range of validity of the Wigner law.

The relative photodetachment cross section of ${\mathrm{Al}}^{\mathrm{\ensuremath{-}}}$ has been measured in the wavelength range 2420--2820 nm (0.440--0.512 eV), using a coaxial ion-laser beams apparatus, in which a 2.98-keV ${\mathrm{Al}}^{\mathrm{\ensuremath{-}}}$ beam is merged with a beam from an F-center laser. The cross-section data near the $^{3}$${\mathit{P}}_{0,1,2}$${\ensuremath{\rightarrow}}^{2}$${\mathit{P}}_{1/2,3/2}$ photodetachment threshold have been fitted to the Wigner threshold law and to the zero-core-contribution theory of photodetachment. The electron affinity of aluminum was determined to be 0.44094(+0.00066/-0.00048) eV, after correcting the experimental threshold for unresolved fine structure in the ground states of ${\mathrm{Al}}^{\mathrm{\ensuremath{-}}}$ and Al. The new measurement is in agreement with the best previous measurement (0.441\ifmmode\pm\else\textpm\fi{}0.010 eV) and is 20 times more precise. The Wigner law agrees with experiment within a few percent for photon energies within 3% of threshold. A proposed leading correction to the Wigner law is discussed. \textcopyright{} 1996 The American Physical Society.

Period doubling and quasi-periodicity in additive-pulse mode-locked lasers

We have observed period doubling and quasi-periodicity in an additive-pulse mode-locked F-center laser. Experiments show that period doubling is often present even though standard diagnostics such as pulse autocorrelations and spectra give no indication of it. Numerical simulations show that the period doubling is associated with strong pulse reshaping.

Direct and indirect detection of liquid chromatography by infrared thermal lens spectrometry

A novel, sensitive and universal detector for liquid chromatography has been developed. This detector is based on the measurement of infrared absorption of effluents by the thermal lens effect. In this instrument, the chromatographic effluent was excited by infrared radiation derived from a solid-state tunable F-center laser. The heat generated as a consequence of the sample absorption of the infrared radiation was measured by a He-Ne monitoring laser whose beam was collinearly overlapped with the excitation infrared beam in the sample. The technique is universal because it can be used for the detection of compounds which have absorption in the infrared region (by direct detection) as well as non-absorbing compounds (by indirect detection). The sensitivity of the technique is directly proportional to the excitation laser power, and with the used of laser power of only 4.5 mW, the technique is at least ten times more sensitive than corresponding absorption detectors. A detection limit of picograms was achieved for phenol and its chloro substituents.

Photofragment vibrational, rotational, and translational distributions for N2–HF (v=1)

Two new methods have been developed for studying the vibrational predissociation dynamics of weakly bound complexes at the state-to-state level. The first is based upon the use of two F-center lasers, one to photodissociate the complex by single quantum state excitation and the second to probe the final states of the resulting fragments. These pump–probe experiments are carried out with angular resolution of the photofragments, giving information on the associated vibrational, rotational and translational distributions. The second method makes use of a dc electric field to orient the parent molecules prior to photodissociation. In this way the two fragments recoil to different sides of the apparatus, allowing us to detect them independently. These experiments also provide us with valuable information on the nature of the photofragment angular distribution in the molecule fixed frame. The present application is to the study of the vibrational predissociation dynamics of N2–HF (v=1). A combination of the two methods has enabled us to determine that the complex dissociates via two distinctly different channels with similar probability, namely one that produces vN2= 1, jHF=7 and the other vN2= 0, jHF=12. The rotational distribution of the N2 fragment is also determined by fitting the angular distributions. Conservation of energy is used to determine the dissociation energy of the complex (D0) to be 398±2 cm−1.

Indirect Amplitude Stabilization of a Tunable Laser through Control of the Intensity of a Pump Laser by an Electro-Optic Modulator

A novel method has been developed to stabilize the intensity of a tunable laser. In this method, the tunable laser is amplitude-stabilized indirectly by controlling the intensity of the pump laser through an electro-optic modulator placed between the pump and the tunable lasers. A small portion of the tunable laser beam was split into a reference photodiode to provide a reference signal for the feedback driver to drive the Pockels cell. Any fluctuation in the intensity of the tunable laser is compensated for by varying the intensity of the pump laser through the feedback driven Pockels cell. Results obtained on the Ti-sapphire laser pumped by an ion laser demonstrate that up to 100 × reduction in the laser noise level can be accomplished by use of this method. Furthermore, with this method, it is possible to adjust the intensity of the laser to be exactly equal for different wavelengths, and to maintain this level for as long as the stabilization is activated. Applications of this method for different types of tunable lasers, including dye and F-center lasers, are discussed.

Generation of high-power femtosecond pulses near 1.5 mu m using a color-center laser system

The authors have developed a high-power F-center laser system capable of generating femtosecond pulses with energies exceeding 50 pJ at wavelengths near 1.5 mu m. Short pulses (140 fs) from an additive-pulse mode-locked NaCl laser are amplified in a multipass NaCl amplifier at kilohertz repetition rates. The system can generate peak powers approaching 500 MW, with wavelengths tunable from 1.52 to 1.64 mu m. The amplified pulses are used to generate a continuum in various solid media without optical damage. The continuum generated in BaF/sub 2/ is extremely broad, extending from below 400 nm to 3.5 mu m. Construction and operating details are discussed as well as the system's utility for femtosecond measurements in the infrared and high-power experiments.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Spectroscopic evidence for near-resonant intermolecular energy transfer in the vibrational predissociation of acetylene-HX and acetylene-DX (X = Cl, Br and I) complexes

Infrared laser molecular beam spectroscopy has been used to study a number of C 2 H 2 -HX and C 2 H 2 -DX complexes. In all cases the asymmetric acetylenic stretch is excited by using an F-center laser while the optothermal method is used to detect the resulting decrease in molecular beam intensity caused by vibrational predissociation of the complex

Theory of electronic defects: Applications to MgO and alkali halides

The topic of electronic defects is introduced with the example of a defect electronic transition, and its role in the F-center lasers. Predictive and methodological objectives of a theory of electronic defects are briefly examined, and general features of a theoretical model of point defects are introduced. The specific theoretical and computational elements of a method bearing the acronym ICECAP are described. They consist of: (i) a quantum molecular defect cluster described in unrestricted Hartree-Fock self-consistent field approximation with Rayleigh-Schrödinger many-body perturbation theory correlation correction; (ii) a classical embedding crystal lattice represented by the shell model; (iii) a treatment of the cluster-lattice boundary condition derived from the Hartree-Fock description of the entire crystal, and including the Kunz-Klein localizing potential; (iv) the static lattice approximation; (v) atomic core pseudopotentials as an option; and (vi) the user-friendly program ICECAP that incorporates the preceding elements in a consistent variational treatment of the total defect lattice. Applications of the method to a number of systems are introduced with a description of several theoretical features that will arise, namely electronic localization and charge transfer, and basis-set and ion-size effects. Details are then given of analyses of the ground states of F centers and hydrogen in MgO and of the V k center in LiF, the optical properties of Cu + in NaF and of the F + center in MgO, and the derivation and application of inter-ionic potentials for impurities and for perfect lattices. Finally, some comments are given regarding short-term and long-term application and development of the method.

Thermal optical bistability in InGaAs/InAlAs MQW etalons at 1.5 mu m wavelength

The authors report on the fabrication of devices assembled in one unit and the observation of thermal optical bistability with low threshold and high reproducibility using the low-energy side of a 1.54- mu m etalon peak. The devices are InGaAs/InAlAs multi-quantum-well (MQW) nonlinear etalons, whose finesse of 20 was obtained by using a SiO/sub 2//TiO/sub 2/ high-reflectivity coating. The authors also show that the nonlinear characteristics depend on the wavelength of the incident light detuned from the etalon peak. The relation between optical bistable characteristics and incident beam wavelength is studied in detail using a tunable F-center laser. >

A simple F-center laser spectrometer for continuous single frequency scans

We report a simple and novel scheme for continuous, single frequency scanning of a commercial F-center laser without any computer interfacing. The scheme utilizes galvo tuning of the cavity length with intracavity CaF 2 Brewster plates with servo loop control of the intracavity etalon. This permits continuous tuning of the F-center frequency over 0.8 cm −1 under complete manual control, as well as arbitrarily long, concatenated scans, and trivial interfacing to a data acquisition system. This scanning spectrometer operation is demonstrated on direct absorption of atomic bromine.

Improved mode locking of an F-center laser with a nonlinear nonsoliton external cavity

We demonstrate that solitons are not essential for the operation of the soliton laser. The external cavity employed contains an optical fiber with negative group-delay dispersion and therefore does not support bright solitons. Thus the improved mode locking cannot be attributed to the injection of an N = 2 soliton.

Vibrational state mixing of individual rotational levels in 1-butyne near 3333 cm−1

The high resolution infrared spectrum of jet-cooled 1-butyne is reported for the acetylenic C–H stretch. The absorbance of an F-center laser beam is enhanced by a multiple reflection cell surrounding the free jet. Although the spectrum in the gas phase (300 K) is a featureless rotational contour, at 3 K sharp features are observed. For J′=0, 1, 2, the K′a =1 lines are spectroscopically well-behaved, but the Ka =0 lines are split into multiplets. Analysis of the K′a =1 lines gives the constants (B′+C′)/2=0.143 82(11) cm−1, (B′−C′)/2=0.007 20(11) cm−1, and ν0+A′−A″=3333.0182(9) cm−1. The quantities in parentheses are two standard deviations in units of the last digit. It is concluded that the Ka =0 levels are mixed with the bath of background vibrations by anharmonic matrix elements the order of 0.01 cm−1.

Laser determinations of ‘‘hot band’’ quantum yields: Br*(2P1/2) formation in the continuum absorption of Br2 at 510–550 nm

Absolute quantum yields of Br* in the photodissociation of thermally excited vibrational levels of Br2 are measured by transient gain vs absorption probing of the Br*(2P1/2)–Br(2P3/2) transition at 2.7μm with an F-center laser. An etalon-narrowed pulsed dye laser with a linewidth of 0.04 cm−1 is used to excite continuum regions between the bound Br2(B–X) transitions of isotopic Br2(∼81% 81Br2) at selected wavelengths between 510 and 550 nm. The threshold wavelength for production of Br*+Br from v″=0, J″=0 is 510.7 nm. At 297 K the Br* yields vary from 85% at 510 nm to 12% at 550 nm. Absorption coefficient data measured with the laser together with the Br* quantum yields at both 297 and 373 K enable a direct determination of the B-state continuum absorption. The data are used to determine the individual contributions of the B 3Π(0+u), 1Π(1u) and A 3Π(1u) states to the continuum absorption in this spectral region. Good agreement is obtained between the measured B-state absorption coefficients and those calculated from the potentials of Le Roy et al., which were obtained by a fit to the total absorption data of Passchier et al. Deviations from Le Roy’s fit at very long wavelengths suggest an adjustment of the inner turning point of the B-state potential near the Br*+Br dissociation limit. The A-state continuum band has a peak absorption coefficient of ∼12ℓ mol−1 cm−1 at around 530 nm.

The structure of the carbon dioxide dimer from near infrared spectroscopy

An F-center laser–molecular beam spectrometer has been used to obtain a sub-Doppler resolution infrared spectrum of the carbon dioxide dimer. The vibrational mode investigated in this study corresponds to the ν1+ν3 combination mode of the monomer located at 3716 cm−1. A qualitative assignment of the spectrum shows unambiguously that the equilibrium structure of the dimer is the slipped parallel, rather than the T-shaped, geometry. The observed spectrum cannot be fit to within experimental error using conventional asymmetric rotor formalism. This may be due to a number of factors such as Fermi resonance between the upper state levels of the band and nearby levels of the dimer, such as seen in the monomer, or it could arise from tunneling effects arising from the two large amplitude motions in the dimer.

Transient gain-versus-absorption laser probing of spin-orbit states, kinetics and dynamics

A tunable F-center laser and a diode laser are used to probe the spin-orbit populations of excited (2P1/2) and ground (2P3/2) state Br and I atoms at 2714 nm and 1315 nm, respectively. The technique uses pulsed laser photolysis and time-resolved cw laser probing of the gain or absorption signals from the coupled pair of states in the halogen atoms. Transient laser gain-versus-absorption measurements provide highly accurate quantum yields of the spin-orbit states in the photodissociation of molecules such as Br2, IBr, CH3I, ICN and C3F7I. In Br2, the continuum yields that result from absorption of vibrationally excited states are investigated as a function of temperature to determine the contributions to the absorption of individual upper excited states. The process of collisional dissociation upwards in energy, termed “collisional release”, is investigated with single vibronic state excitation. The high resolution of the probe lasers allows Doppler velocity effects to be studied, and differing reactivity of the spin-orbit excited and ground state atoms is explored. The high sensitivity and excellent time resolution of the transient gain vs. absorption technique provides a powerful way to study numerous kinetic phenomena with exceptional detail

The vibrational predissociation lifetime of the HF dimer upon exciting the ‘‘free-H’’ stretching vibration

A computer controlled F-center laser has been used, in conjunction with a molecular beam apparatus employing optothermal detection, to obtain an accurate measurement of the vibrational predissociation lifetime of the HF dimer following ν1 vibrational excitation. The lifetime obtained in this study is 24±2 ns. This is to be compared with a lifetime for ν2 excitation of 1.0±0.1 ns.