Tunable Laser Radiation Research Articles

Wavelength dependence of the photoassociation of Kr(4p 61S 0)−F(2p 52P) collision pairs

The excitation spectrum for the photoassociation (bound←free absorption) of thermalized pairs of Kr(4p 6 1S 0) and F(2p 5 2P) atoms in Kr/F 2 gas mixtures at 300 K has been recorded in the ultraviolet (208⩽λ⩽250 nm). By photodissociating F 2 in the presence of Kr and subsequently photoexciting Kr—F collision pairs in the ground state thermal continuum with tunable laser radiation, deeply modulated Franck—Condon structure associated with bound←free transitions terminating on the lowest 26 vibrational levels of the B 2Σ ion pair state of the KrF molecule was observed. Since the KrF ground state interaction potential is repulsive, photoassociation occurs in a narrow (Δ R≈0.7 Å) Franck—Condon region and structural information regarding the upper and lower state potentials, as well as the dependence of the electronic transition moment on internuclear separation, can be extracted from the spectrum. The Kr—F system is presented here as an example of the effectiveness of photoassociation as a tool with which one can determine the chemical, structural and optical properties of small excimer molecules that are otherwise difficult to access experimentally.

Electronic spectra of N+2–(He)n (n=1, 2, 3)

Ionic clusters of nitrogen and helium have been investigated by recording their electronic spectra in the near UV. Structured bands belonging to 14N+2 –He, 14N+2–(He)2, 14N+2–(He)3, and 15N+2 –He have been measured between 390 and 392 nm close to the N+2 B 2Σ+u←X 2Σ+g band origin. Spectra were obtained by exciting mass selected cluster cations with tunable laser radiation and recording the photodissociation cross section as a function of wavelength. The data support the hypothesis that the He– – –N+2 interaction potential has only a small barrier to internal rotation in both the X and B electronic states. A lower estimate for the dissociation energy of the N+2 –He cluster of 101 cm−1 is inferred.

Photodissociation spectroscopy of benzene cluster ions: (C6H6)+2 and (C6H6)+3

The photodissociation of size-selected benzene cluster ions, (C6H6)+m+hω→(C6H6)+n+ (m−n)C6H6, has been investigated in the 410–750 nm wavelength range using tunable dye laser radiation. The measurements were performed using a tandem mass spectrometer [a linear time-of-flight (TOF)/reflectron TOF] combined with multiphoton ionization (MPI) for ion preparation. Only C6H+6 was detected as a photofragment of (C6H6)+2 and (C6H6)+3, while both C6H+6 and (C6H6)+2 fragments were observed in the case of (C6H6)+4 photodissociation. Photodissociation spectra, i.e., photofragment yield spectra as a function of wavelength, of (C6H6)+m (m=2,3) were obtained. Two local excitation bands of (C6H6)+m were seen in this region and assigned to the C(A2u)←X(E1g) and the B(E2g)←X(E1g) transitions of a C6H+6 unit in the clusters. The origin of the B←X transition of (C6H6)+2 and (C6H6)+3 was redshifted relative to that of C6H+6 by about 1400 and 2400 cm−1, respectively, while the C←X bands of (C6H6)+2 and (C6H6)+3 were seen at the same wavelengths of 440 nm. Possible structures for the cluster ions are discussed based on the spectral shifts.

Noncritical detection of tunable CO2 laser radiation into the green by upconversion in silver thio-gallate

Tangential near-noncritical phase matching is reported for upconversion in silver thiogallate (AgGaS2) using a dye laser as pump. A large acceptance angle for noncollinearity as desirable from a device view point is realised. Weak temperature tunability is also exploited to realise noncriticality.

Investigation of internal rotor dynamics of NeDCl and ArDCl via infrared absorption spectroscopy

The van der Waals complexes, NeDCl and ArDCl, are produced in a slit jet supersonic expansion and observed via direct absorption of tunable mid-infrared Pb-salt diode laser radiation. For the NeDCl complex, the DCl stretch fundamental [ν0=2091.3717(4) cm−1 ] and the DCl Σ and Πe, f bend combination bands [ν0=2099.5760(4) and 2104.9465(4) cm−1, respectively] are observed. The DCl stretch fundamental and Πe, f combination band are observed for ArDCl at 2089.4180(2) and 2117.4443(3) cm−1, respectively. The relative fundamental vs bend combination band intensity distributions are very different for the two complexes. The ArDCl fundamental to Π bend combination band intensity ratio is 4:1, whereas for NeDCl the corresponding ratio is 1:8. This anomalous intensity pattern for NeDCl and the proximity of the bend combination bands to the DCl R(0) line indicate that the DCl diatomic is exhibiting nearly free rotation within this complex, compared to more restricted librational motion of DCl in ArDCl. Strong Coriolis interactions between Πe and Σ bend states are observed for both complexes and analyzed quantitatively for NeDCl. The observed NeDCl and ArDCl absorption linewidths are only slightly larger than the instrumental limit determined from nearby OCS monomer absorptions in the slit jet, but the differences are not of high statistical significance. This FWHM of the observed transitions dictates a rigorous lower limit to the vibrational predissociation lifetime of 3 ns. Experimentally determined rotational constants, vibrational frequencies, and relative intensities are compared to predictions based on existing empirical potential surfaces.

Remote photoacoustic measurements in aqueous solutions using an optical fiber

A photoacoustic spectrometer was developed for remote optical absorption measurements in aqueous solutions using an 85-m optical fiber to deliver pulsed tunable dye laser radiation to a sample cuvette located in a glove box. The spectrometer was tested using aqueous solutions of praseodymium ions. Beer’s law was verified down to a concentration of 8×10−6M for an equivalent absorptance of 3.2×10−5 cm−1.

Exciton-polaritons in nonlinear optical phenomena in semiconductors: An overview of major developments

In the past decade, the availability of techniques for generating very intense tunable ultrashort-pulse laser radiation in the picoseconds and femtoseconds range have made it possible to use time-resolved pump probe techniques for studying the dynamics of the nonlinear response of bulk semiconductors and semiconductor quantum well structures to intense EM beams. Moreover, as a result of interest in nonlinear optical phenomena that might be used in optical memory, logic and switching applications, there has been an intensification of research on the effects of intense EM beams on optical properties in the vicinity of the exciton resonances and optical absorption edges of semiconductors. In this article we present an overview of key experimental and theoretical developments that underlie our understanding of the role played by exciton-polaritons in nonlinear optical phenomena in semiconductors and, in particular, the role played by exciton-polaritons in the nonlinear optical effects, including the optical Stark effect, which result from the virtual excitation of excitons, continuum electron-hole pairs and biexcitons by an ultrashort pulse of intense EM radiation at frequencies below the exciton resonance, and which exist only during the time of the pulse.

Resonant two-photon dissociation of Ni 2+

Ni 2 + ions generated in a pulsed high-temperature plasma and cooled by supersonic expansion are photodissociated with tunable visible laser radiation. Resonant bound-bound absorption in the isolated ion is detected by the observation of Ni 2 + → Ni + + Ni sequential two-photon dissociation. Progressions of vibronic bands with partially resolved rotational structure belonging to several distinct electronic transitions in Ni 2 + have been observed in the interval between 16250 and 23500 cm −1. The appearance and preliminary analysis of this resonant two-photon dissociation (R2PD) spectrum will be discussed.

Temperature-tunable second-harmonic generation in zinc germanium diphosphide

Second-harmonic generation of tunable CO2 laser radiation is reported and analyzed in ZnGeP2 both by angular and temperature tunings. Temperature tuning can thus be advantageously employed in this crystal for nonlinear devices.

VUV laser spectroscopy of Xe 2 in the C′ 1 u state

Xe 2 molecules in a supersonic free jet have been excited to the C′ 1 u state with tunable and monochromatic VUV laser radiation generated by four-wave difference frequency mixing in Kr. From the vibrationally resolved excitation spectrum, the following spectroscopy constants are derived for the C′ 1 u state: T′ e = 77240.6, ω′ e = 16.2, ω eχ′ e = 1.1, and D′ e = 140 cm −1. It was also found that the radiative lifetimes of the ν′ = 0 and 1 states of the C′ 1 u state were 4.3±0.5 and 3.8±0.6 ns, respectively.

On the nature of NiAr +

NiAr + ions generated in a supersonic expansion of Ni + with a 1% Ar/He carrier gas are injected into a tandem mass spectrometer and photodissociated with visible tunable laser radiation. A photofragmentation feature observed at 17984 cm − is believed to correspond to the dissociation threshold of NiAr + to excited state ( 2F 7 2) Ni + and ground state Ar. This assignment yields a prediction of 0.55 eV for the binding energy ( D 0 0) of NiAr +. The 58NiAr +/ 60NiAr + isotope shift of this spectral feature indicates the vibrational frequency in the ground state of NiAr + to be 235± 50 cm −. Thus NiAr + is a surprisingly “stiff” and strongly bound diatomic, discussed in light of a simple electrostatic (charge-induced-dipole) model of the internuclear forces in this class of molecules.

High power tunable UV laser radiation produced by second-harmonic generation in β-BaB2O4

High power tunable UV laser radiation from 2750Å to 3200Å has been obtained by second-harmonic generation in a β-BaB2O4 crystal. A maximum frequency doubling efficiency of 41% and a UV output power of 3.7MW have been obtained.

The infrared spectrum of calcium oxide in the gas phase

The first gas-phase infrared spectrum of an alkaline earth metal oxide is reported. CaO was produced in a fast flow system by oxidation of hot metal vapor with nitrous oxide. Eight transitions of the fundamental band of calcium oxide have been observed with tunable diode laser radiation. A value of 722.3976(14) cm −1 for the band center has been obtained.

Tunable laser radiation, nonlinear absorption, and “anti-stokes” luminescence of color centers in α-Al2O3

This work investigates the generation of laser radiation by color centers of leucosapphire, nonlinear absorption, and the excitation mechanism of UV-luminescence. It is shown that UV-luminescence from color centers, excited in the 0.68 μm absorption band, is caused by a two-staged excitation mechanism. The optical characteristics of the color centers are measured and calculation is made of the variation of the absorption coefficient with laser pulse's duration.

Gas discharge modulation using fast electronic switches: application to HeH+

A high-voltage modulator based on metal oxide semiconductor field effect transistors (MOSFETs) has been developed. It has been employed for on-off modulation of gas discharges as well as for reversing the polarity of the electrodes in a dc discharge. As a first application to spectroscopy, new results for the vibration-rotation bands of HeH +, measured with tunable diode laser radiation, are presented.

Evaluation of stimulated Raman scattering of tunable dye laser radiation as a primary excitation source for exciting atomic fluorescence in an inductively coupled plasma

Radiation generated by stimulated Raman scattering (SRS) in H/sub 2/ at liquid N/sub 2/ temperatures was evaluated as a tunable, sharp-line, ultraviolet laser source for the excitation of atomic fluorescence of those elements, such as As, Se, Te, and Zn, whose resonance excited states are usually more than 40,000 cm/sup -1/ above the ground state. To populate these states requires primary radiation of wavelengths that are shorter than can be provided by laser-frequency doubling techniques. These wavelengths are within the range that can be reached through stimulated Raman scattering of dye-laser primary radiation. In the studies described, a YAG pumped, dye-laser source provided the primary radiation that was scattered in the SRS cell into Stokes and anti-Stokes components. Data on the analytical figures of merit are presented and compared with those obtained by conventional inductively coupled plasma atomic emission spectroscopy and hollow-cathode lamp excited atomic fluorescence spectroscopy.

Wavelength dependence of laser-induced sputtering from the (111) surface of BaF2

Using blue tunable pulse laser radiation of low fluence, we have investigated laser-induced sputtering from cleaved BaF2 (111) surfaces under ultrahigh vacuum conditions. Time correlated with the laser pulses the positive ions Ba+, Ba++, (BaF)+ and F+ were observed. Practically no negative ions were found. Neutral atomic fluorine (F 0) was desorbed abundantly. A pulse correlation of F 0 as well as the relative amount of F 0 and F+ could not be established at this stage. The emission yield of all positive ions as well as of F 0 was strongly wavelength dependent and showed a broad resonance around 2.9 eV.

Vibrational spectroscopy of adsorbates using a tunable diode laser

Single-reflection infrared reflection-absorption spectra (IRRAS) of adsorbates on surfaces have been obtained using a tunable diode laser radiation source. Surface sensitivity is achieved through the use of polarization modulation, and the high brightness of the laser light source has made possible the observation of vibrational modes at energies much lower than those accessible with conventional blackbody sources. Results are presented for the chemisorption of oxygen on polycrystalline aluminum showing surface (580 cm −1) and subsurface (890 cm −1) Al-O stretching modes at sub-monolayer coverage.

Generation of nuclear-spin-polarized protons by resonant ionization of hydrogen atoms.

An atomic beam of H atoms was crossed by a pulsed beam of narrow bandwidth ( approx.700 MHz) circularly polarized tunable vuv laser radiation at lambda = 121.6 nm. The atoms were excited into a single magnetic substate in the 2p SP/sub 1/2/ level and thus polarized. This polarization is transferred via hyperfine interaction to the nucleus. A second tunable uv laser served to ionize these atoms, thereby producing nuclear-spin-polarized protons with, theoretically, 100% degree of polarization. By changing the helicity of the circularly polarized vuv laser with a quarter-wave plate, the polarization direction of the protons is changed.

Rotational state selective photoionization of the H2 molecule from B 1Σ+u(v ′=0,3) states

Selective photoionization from single rotational states of the B 1Σ+u(v′=0 and 3) excited state of the H2 molecule has been investigated with fixed frequency and tunable laser radiation around λ=266 nm. State selection of the B-state levels was achieved by tunable VUV laser excitation around λ=110 and 106 nm, respectively. The ionization cross section has been determined for different states, and found to be critically dependent upon the initial rotational quantum state and to show pronounced spectral structure with the ionization wavelength. At λI=266.05 nm the cross section varies between 0.23×10−18 and 1.2×10−18 cm2, depending on the initial state.