Four-wave Sum Frequency Mixing Research Articles

Resonance ionisation mass spectrometry of krypton and its applications in planetary science

A new resonance ionisation time-of-flight mass spectrometer for determining krypton isotope ratios in extraterrestrial samples is presented. Laser heating is used to extract gas from mg-size samples. A cryogenic sample concentrator is employed. Atoms continuously condense on a 75 K stainless steel substrate at the back plate of a Wiley-McLaren laser ion source from where they are desorbed by a pulsed 1064 nm laser and resonantly ionized in the plume. A three-colour (116.5 nm, 558.1 nm and 1064 nm) excitation scheme is used. Tuneable coherent Vacuum Ultraviolet (vuv) radiation near 116.5 nm is generated by four-wave sum frequency mixing of 252.5 nm and 1507 nm pulsed dye laser beams in a binary mixture of negatively and positively dispersive gases (Xe and Ar). Isotope effects have been observed that reduce the reproducibility of isotope ratio measurements between odd-mass, non-zero nuclear spin isotopes and even-mass, zero nuclear spin isotopes. This can be minimised and stabilised by controlling the laser fluences, experimental geometry, and the population of the magnetic sub-levels of the excited atomic states used in the ionisation process. Once stability is achieved, sample-standard bracketing (during which the known isotope ratios of a standard are determined before and after the measurements of the sample under the same conditions) allows precision and reproducibility of \(\sim \)1 % for the major isotope ratios to be achieved in samples \(\sim 10^{6}\) krypton atoms. Detection limits of \(<1000\) atoms/isotope have been demonstrated, ratios of \(^{81}\)Kr in meteorites have been made with \(\sim \)5–10 % precision. Applications of the instrument in various areas of planetary science are also discussed.

Generation of multicolor vacuum ultraviolet pulses through four-wave sum-frequency mixing in argon

We demonstrate efficient generation of multicolor vacuum ultraviolet pulses with excellent mode quality through ${\ensuremath{\chi}}^{(3)}$-based four-wave sum-frequency mixing and third-order harmonic generation of 400- and 267-nm femtosecond laser pulses in argon gas. The ${\ensuremath{\chi}}^{(3)}$-based nonlinear optical processes were optimized with appropriate control of gas pressure and group velocity delay between the driving pulses. Furthermore, the pulse breakup effects were observed for tightly focused ultraviolet pulses.

A reliable cw Lyman-α laser source for future cooling of antihydrogen

We demonstrate a reliable continuous-wave (cw) laser source at the 1 S–2 P transition in (anti)hydrogen at 121.56 nm (Lyman-α) based on four-wave sum-frequency mixing in mercury. A two-photon resonance in the four-wave mixing scheme is essential for a powerful cw Lyman-α source and is well investigated.

Continuous-wave Lyman-α generation with solid-state lasers

A coherent continuous-wave Lyman-alpha source based on four-wave sum-frequency mixing in mercury vapor has been realized with solid-state lasers. The third-order nonlinear susceptibility is enhanced by the 6(1)S - 7(1)S two-photon resonance and the near 6(1)S-6(3)P one-photon resonance. The phase matching curve for this four-wave mixing scheme is observed for the first time. In addition we investigate the two-photon enhancement of the Lyman-alpha yield and observe that the maxima of Lyman-alpha generation are shifted compared to the two-photon resonances of the different isotopes.

Tunable VUV light generation for resonance ionization mass spectrometry of Krypton

Tunable coherent VUV radiation from 115.8 to 116.9nm has been produced by non-linear four-wave sum frequency mixing in a xenon–argon mixture. 116.5nm light generated by this means has been used as the first step in a three color, doubly resonant ionization scheme for Kr. In the process of validating the system the xenon refractive index per atom (STP) at 116.5nm has been determined to be (n(Xe)−1)/NXe=−6.8(±0.8)×10−23cm3.

Mass-Analyzed Threshold Ionization Study of Vinyl Chloride Cation in the First Excited Electronic State Using Vacuum Ultraviolet Radiation in the 107−102.8 nm Range

Vibrational spectrum of vinyl chloride cation in the first excited electronic state, A2 A', was obtained by one-photon mass-analyzed threshold ionization (MATI) spectroscopy. Use of an improved vacuum ultraviolet radiation source based on four-wave sum frequency mixing in Hg resulted in excellent sensitivity for the MATI signals. From the MATI spectrum, the ionization energy to the A2 A' state of the cation was determined to be 11.6667 +/- 0.0006 eV. Nearly complete vibrational assignment for the MATI peaks was possible by utilizing the vibrational frequencies and Franck-Condon factors calculated at the DFT and TDDFT/B3LYP levels with the 6-311++G(3df,3pd) basis set. Geometry of the cation in the A2 A' state was determined by Franck-Condon fitting of the MATI spectrum.

N(4S) Formation following the 193.3-nm ArF Laser Irradiation of NO and NO2 and Its Application to Kinetic Studies of N(4S) Reactions with NO and NO2

Formation of the ground-state nitrogen atom, N((4)S), following 193.3-nm ArF laser irradiation of NO and NO(2) was detected directly by a technique of laser-induced fluorescence (LIF) spectroscopy at 120.07 nm. Tunable vacuum ultraviolet (VUV) laser radiation around 120.07 nm was generated by two-photon resonance four-wave sum frequency mixing in Hg vapor. Photoexcitation processes of NO and NO(2) giving rise to the N((4)S) formation are discussed on the basis of the Doppler profiles of the nascent N((4)S) atoms produced from the photolysis of NO and NO(2) and the photolysis laser-power dependence of the N((4)S) signal intensities. Using laser flash photolysis and vacuum ultraviolet laser-induced fluorescence detection, the kinetics of the reactions of N((4)S) with NO and NO(2) have been investigated at 295 +/- 2 K. The rate constants for the reactions of N((4)S) with NO and NO(2) were determined to be (3.8 +/- 0.2) x 10(-11) and (7.3 +/- 0.9) x 10(-12) cm(3) molecule(-1) s(-1), respectively, where the quoted uncertainties are 2sigma statistical uncertainty including estimated systematic error.

Mass-analyzed threshold ionization study of vinyl bromide cation in the first excited electronic state using vacuum-ultraviolet radiation generated by four-wave mixing in Hg

The vibrational spectrum of the vinyl bromide cation in the first excited electronic state A 2A' was obtained by one-photon mass-analyzed threshold ionization (MATI) spectroscopy. The use of an improved vacuum-ultraviolet radiation source based on four-wave sum frequency mixing in Hg resulted in excellent sensitivity for MATI signals. From the MATI spectrum, the ionization energy to the A 2A' state of the cation was determined to be 10.9150+/-0.0006 eV. Nearly complete vibrational assignments for the MATI peaks were possible by utilizing the vibrational frequencies and Franck-Condon factors calculated at the density-functional theory (DFT) and time-dependent DFT/B3LYP levels with the 6-311+G(df,p) basis set.

Vacuum ultraviolet mass-analyzed threshold ionization spectroscopy of benzene: Vibrational analysis of C6H6+ and C6D6+ in the B̃ 2E2g state

Vibrational spectra of C6H6+ and C6D6+ in the first excited electronic state, B̃ 2E2g, have been measured by one-photon mass-analyzed threshold ionization spectroscopy using coherent vacuum ultraviolet radiation generated by four-wave sum frequency mixing in Hg. Ionization energies to C6H6+ and C6D6+ in the B̃ 2E2g state have been determined to be 11.4897±0.0006 and 11.5235±0.0006 eV, respectively. Vibrational spectra were much simpler than and different from those of the same state recorded by the gerade photo-induced Rydberg ionization technique reported previously. For some vibrations not much affected by the Jahn–Teller effect, assignments were possible by comparing with the calculated results. Assignments for the Jahn–Teller components were not definite, or rather arbitrary, even though decent fits were possible through multimode Jahn–Teller calculations.

Development of a coherent vacuum ultraviolet source at 104–108 nm generated by four-wave sum frequency mixing in Hg vapor

A continuously tunable vacuum ultraviolet (VUV) light source in the 104–108 nm range has been developed by utilizing four-wave sum frequency mixing in Hg vapor. The windowless Hg cell designed uses a conelike glass capillary which transmits most of VUV generated while it restricts the flow of Hg and buffer gas. Usable power of about 109 photons/pulse (2 nJ/pulse) was obtained across the spectral region covered. The light source could be run for at least a month without maintenance and did not seem to contaminate the main apparatus.

Polarization properties of coherent VUV light at 125 nm generated by sum-frequency four-wave mixing in mercury

The polarization of the VUV light generated by four-wave sum-frequency mixing process ω4=2ω1+ω2 in mercury vapor at room temperature is analyzed in detail. Due to the specific two-photon transition used to enhance the nonlinear process, the polarization of the VUV wave is shown to be identical to the polarization of the wave at the frequency ω2. In particular, circularly polarized VUV is observed with degree of circular polarization exceeding 0.99.

Solar blind chemically vapor deposited diamond detectors for vacuum ultraviolet pulsed light-source characterization

A major difficulty in characterizing vacuum ultraviolet (VUV) radiation produced by harmonic generation or four-wave sum frequency mixing arises in differentiating between the desired VUV signal and the remaining fundamental pump laser beam. To overcome this problem, visible and near UV blind VUV detectors, made from natural and synthetic diamond, have been developed. Such detectors have been used to characterize coherent VUV pulses (λ=125 nm, pulse duration at full width half maximum (FWHM) τFWHM∼7 ns) generated by resonance-enhanced four-wave sum mixing in mercury vapor. They allow full characterization of the intensity profile of the VUV pulses, without any significant parasitic signal from simultaneous stray light irradiation at λ=313 nm. Detectors were fabricated exhibiting response times of less than 70 ps at FWHM, corresponding to the lowest response time obtainable with a 7 GHz bandwidth single-shot oscilloscope.

A very convenient setup to generate intense VUV coherent light at 125 nm with use of nonlinear effects in mercury vapor at room temperature

Using only one dye laser, efficient generation of VUV radiation (10/sup 13/ photons/pulse) is demonstrated at 125.140 nn and 125.053 nm by four-wave sum-frequency mixing in a room-temperature mercury vapor. The emission at 125.053 mn, which is out of two-photon resonance, but near three-photon resonance, has been observed for the first time and is carefully analyzed. In particular, numerical calculations have been carried out for Gaussian pump beams, taking into account absorption of VUV photons and optical Kerr effect. The results of the calculations reproduce the VUV emission lineshape and power saturation effects measured in the experiment. >

Observation of the Stark effect in autoionizing Rydberg states of Ar with a multichannel quantum-defect analysis

The Stark spectra of autoionizing Rydberg states of Ar converging to the second ionization limit 3p5 2P1/2 have been investigated, experimentally and theoretically, as a function of electric field strength, in the range 15-2000 V cm-1. The levels with effective principal quantum number v=12-19, are excited by single-photon absorption from the ground state using a coherent extreme ultraviolet laser source (bandwidth approximately 1.0 cm-1) generated by four-wave sum-frequency mixing of the outputs of two pulsed dye lasers; a detailed Stark map is obtained for the first time. A multichannel quantum-defect-theory analysis of the field-induced-state mixing is carried out, reproducing positions, widths and shapes of the experimentally observed features. In addition, this analysis provides new insight into the dynamical decay of the autoionizing states.

Suppression of a four-photon resonance by four-wave mixing near an intermediate three-photon resonance.

We use resonance ionization involving two colors in single-pass and retroreflected geometries to study the suppression of excitation of a four-photon resonance by an interference involving four-wave sum-frequency generation occurring near an intermediate three-photon resonance. The 6[ital p][1/2]([ital J]=0) state of xenon is excited by three blue photons plus one infrared photon. One beam, focused to [ital I][sub 1][congruent]7[times]10[sup 9] W/cm[sup 2], is nearly three-photon resonant with the 6[ital s][3/2]([ital J]=1) resonance; the second, unfocused beam ([ital I][sub 2][congruent]2[times]10[sup 7] W/cm[sup 2]) is tuned to maintain exact four-photon resonance with the 6[ital p][1/2]([ital J]=1) state. One additional blue photon ionizes from the four-photon level. Third-harmonic light is generated by the first, nearly three-photon-resonant beam by four-wave sum-frequency mixing. One third-harmonic photon plus one photon from the second laser form a second excitation pathway to the four-photon level; destructive interference between the two pathways results in a reduction in the observed ionization. The degree of reduction depends on the xenon density and the detuning (both positive and negative) from three-photon resonance. This interference may be overcome by retroreflecting the nearly three-photon-resonant beam to provide an additional excitation pathway which generates no third harmonic.

Generation of XUV radiation by four-wave mixing in CO

Coherent extreme ultraviolet (XUV) radiation was generated by four-wave sum frequency mixing via the B 1Σ + ( v' = 0) state of CO. The XUV intensity was found to be strongly wavelength dependent. The analysis of the spectrum revealed (i) that the XUV intensity was enhanced at high energy side of rotational levels of the probed Rydberg states and (ii) that the observed rotational structure is in accordance with the selection rules holding for ordinary single photon transitions. The results are duscussed in terms of the third-order non-linear processes in isotropic media.

Narrow bandwidth coherent VUV spectroscopy of the Kr 123.58 nm line

Coherent VUV light has been generated with a bandwidth of 0.02−0.04 cm -1 at 80917 cm -1 (123.6 nm) using four-wave sum frequency mixing of a pulse amplified, cw ring dye laser and a single longitudinal mode, pulsed dye laser. This VUV light was used to study the isotope structure of the Kr 123.58 nm line.

VUV laser absorption spectrometer system for measurement of H0 density and temperature in a plasma

A system to determine the density and temperature of ground-state hydrogen atoms in a hydrogen plasma by measurement of the absorption of Lyman-beta or Lyman-gamma radiation is described. The Lyman-series radiation is generated by resonant four-wave sum-frequency mixing in mercury vapor. A wide range of hydrogen atom densities can be measured by employing these two transitions. A sample measurement on a H− ion-source discharge is presented. Extensions to Lyman-alpha and other vacuum-ultraviolet wavelengths are discussed.

Short-Lifetime Measurement of Molecules in the Vacuum Ultraviolet Using High-Resolution Laser Spectroscopy

A measurement system of short radiative lifetimes of molecules at high spectral resolution in the vacuum ultraviolet (VUV) region is developed. Tunable VUV radiation is generated by the four-wave sum-frequency mixing method in Mg vapor. A specially-designed dye-laser system with short pulse duration (less than 2 ns) is used. Individual rotational levels of CO A1Π(v' =0, 1) are excited and the fluorescence decay times are measured down to 5 ns with a spectral resolution of 0.3 cm-1.

Vacuum ultraviolet generation in phase matched carbon monoxide

Resonantly enhanced four-wave sum frequency mixing in phase matched carbon monoxide leads to the generation of continuously tunable over 1200 cm -1 coherent VUV radiation in the 1150 Å range. Experiments allow to determine the VUV absorption cross sections and indices of refraction of CO. Multiphoton photolysis of CO has been observed and confirmed by the VUV absorption in atomic carbon.