Four-wave Difference-frequency Mixing Research Articles

Efficient (∼10%) generation of vacuum ultraviolet femtosecond pulses via four-wave mixing in hollow-core fibers.

We report the generation of the fifth harmonic of Ti:sapphire, at 160 nm, with more than 4 µJ of pulse energy and a pulse length of 37 fs with a 1 kHz repetition rate. The vacuum ultraviolet pulses are produced using four-wave difference frequency mixing in a He-filled stretched hollow-core fiber, driven by a pump at 267 nm and seeded at 800 nm. Guided by simulations using Luna.jl, we are able to optimize the process carefully. The result is a conversion efficiency of ∼10% from the 267 nm pump beam.

Efficient generation of the 7th harmonic of Ti:sapphire (114.6 nm) vacuum ultraviolet pulses with 60 fs duration by non-collinear four-wave mixing in argon.

The recent advances in femtosecond vacuum UV (VUV) pulse generation, pioneered by the work of Noack et al., has enabled new experiments in ultrafast time-resolved spectroscopy. Expanding on this work, we report the generation of 60 fs VUV pulses at the 7th harmonic of Ti:sapphire with more than 50 nJ of pulse energy at a repetition rate of 1 kHz. The 114.6 nm pulses are produced using non-collinear four-wave difference-frequency mixing in argon. The non-collinear geometry increases the phase-matching pressure, and results in a conversion efficiency of ∼10-3 from the 200 nm pump beam. The VUV pulses are pre-chirp-compensated for material dispersion with xenon, which has negative dispersion in this wavelength range, thus allowing almost transform-limited pulses to be delivered to the experimental chamber.

Ring opening dissociation of cationic twisted conformer by conformer-specific cyclopentanone photoionization.

We investigated the dissociation processes of a cationic conformer, induced by conformer-specific photoionization of cyclopentanone (CP) using a one-photon vacuum ultraviolet (VUV) laser pulse of energy in the range 9.24-9.92 eV for a few nanoseconds, generated by four-wave difference frequency mixing in a Kr cell. The adiabatic ionization energy of the CP was accurately determined to be 9.2697 ± 0.0009 eV, based on the VUV photoionization efficiency curve obtained using high-resolution VUV-photoionization time-of-flight (TOF) mass spectroscopy. The constructed potential energy contours, associated with the twisting and out-of-plane motions in the S0 and D0 states, revealed that the ionization energy value corresponded to a twisted conformer with C2 symmetry at the global minimum. Subsequently, the low photon energy above the ionization onset of the twisted conformer in the CP led to C2H4 elimination, producing a C3H4O+ fragment directly prior to CO elimination for the C4H8 + fragment. The appearance energies for the C3H4O+ and C4H8 + were determined to be 9.7068 ± 0.0017 eV and 9.7483 ± 0.0017 eV, respectively, by measuring the fragmentation yield curves for two fragments analyzed in the TOF mass spectra. The formation enthalpy for each fragment ion at 0 K, evaluated using the measured and thermochemical data, enabled the realization of plausible structures for the produced fragment ions. Consequently, based on the results of the quantum chemical calculation on the dissociation processes of the twisted CP cation (t-CP+), we suggest that the fragmentation processes to C3H4O+ and C4H8 + correspond to the methylketene and (E)-2-butene cations, respectively.

A simple resonance enhanced laser ionization scheme for CO via the A1Π state.

We investigate the laser ionization process taking place when the CO molecule is exposed to vacuum ultraviolet (VUV) radiation resonant with the CO A1Π(v = 0) ← X1Σ+(v = 0) transition around 154 nm, along with the ultraviolet (UV) and visible (Red) radiation used to generate VUV by four-wave difference-frequency mixing. By measuring the CO+ ion recoil and a room temperature gas spectrum, it is possible to assign the ionization process as 1 + 1' + 1'' REMPI where the one-photon steps refer to the VUV, UV, and Red radiation, respectively. Resonance enhanced ionization of rotational states around J = 12 arise due to the overlap of the fixed wavelength UV (∼250 nm) with the R band-head of a transition assigned to CO E1Π(v = 6) ← A1Π(v = 0) with a term value of 104 787.5 cm-1. The REMPI process is efficient and polarization sensitive and should be useful in a wide range of studies involving nascent CO.

Carrier-Envelope Phase of Single-Cycle Pulses Generated by Using Four-Wave Difference Frequency Mixing

Carrier-Envelope Phase of Single-Cycle Pulses Generated by Using Four-Wave Difference Frequency Mixing

One-photon mass-analyzed threshold ionization (MATI) spectroscopy of pyridine: determination of accurate ionization energy and cationic structure.

Ionization energies and cationic structures of pyridine were intensively investigated utilizing one-photon mass-analyzed threshold ionization (MATI) spectroscopy with vacuum ultraviolet radiation generated by four-wave difference frequency mixing in Kr. The present one-photon high-resolution MATI spectrum of pyridine demonstrated a much finer and richer vibrational structure than that of the previously reported two-photon MATI spectrum. From the MATI spectrum and photoionization efficiency curve, the accurate ionization energy of the ionic ground state of pyridine was confidently determined to be 73,570 ± 6 cm(-1) (9.1215 ± 0.0007 eV). The observed spectrum was almost completely assigned by utilizing Franck-Condon factors and vibrational frequencies calculated through adjustments of the geometrical parameters of cationic pyridine at the B3LYP/cc-pVTZ level. A unique feature unveiled through rigorous analysis was the prominent progression of the 10 vibrational mode, which corresponds to in-plane ring bending, and the combination of other totally symmetric fundamentals with the ring bending overtones, which contribute to the geometrical change upon ionization. Notably, the remaining peaks originate from the upper electronic state ((2)A2), as predicted by high-resolution photoelectron spectroscopy studies and symmetry-adapted cluster configuration interaction calculations. Based on the quantitatively good agreement between the experimental and calculated results, it was concluded that upon ionization the pyridine cation in the ground electronic state should have a planar structure of C(2v) symmetry through the C-N axis.

Ionization energy of acetone by vacuum ultraviolet mass-analyzed threshold ionization spectrometry

Mass-analyzed threshold ionization (MATI) time-of-flight mass spectrometer using coherent vacuum ultraviolet (VUV) laser generated by four-wave difference frequency mixing (FWDFM) in Kr has been constructed and utilized to obtain the accurate ionization energy of acetone. From the MATI onsets measured from various applied pulsed fields, the ionization energy to the ionic ground state of acetone has been determined to be 9.7074 ± 0.0019 eV.

Terahertz generation based on four-wave mixing difference frequency by resonance-enhanced third-order nonlinear of media

Using nanosecond pulse near-infrared and mid-infrared laser pulses as the pump source, we obtain terahertz wave sources via four-wave difference frequency mixing. From the coupled wave theory, we analyze the four-wave mixing process of GaSe crystal and alkali metal vapor in detail, get the analytical expression of terahertz wave output power, and discuss the conditions for achieving phase matching. By adjusting the pump frequency, the third-order nonlinear polarization of alkali metal vapor is resonance-enhanced. This program offers a new type of high-power terahertz radiation source.

Generation of sub-50-fs vacuum ultraviolet pulses by four-wave mixing in argon

We report on the generation of femtosecond pulses at 160 nm with energies up to 240 nJ at 1 kHz repetition rate and sub-50-fs pulse duration. This pulse energy is a 1-order-of-magnitude improvement compared with previous sub-100-fs sources in this wavelength range. The pulses are generated by four-wave difference-frequency mixing process between the fundamental of a Ti:sapphire laser and its third harmonic in argon. Pulse duration measurements are achieved by pump-probe ionization of Xe gas providing the cross correlation between the fifth harmonic and the fundamental.

Generation and amplification of ultrashort UV pulses via parametric four-wave interactions in transparent solid-state media

We report on efcient generation of the ultrashort ultraviolet light pulses by means of phase-matched four-wave optical parametric amplication and four-wave difference frequency mixing in UV-transparent solids with cubic nonlinearity. Broadband four-wave optical parametric amplication around 330 nm is investigated theoretically and demonstrated experimentally in optically isotropic materials, such as fused silica and CaF2 crystals. The results suggest that powerful broadband ultraviolet pulses as short as 7.5 fs (in the transform limit) could be amplied in the eld of milijoule 1-ps visible pulses. Efcient generation of 1-ps pulses at the 3rd (351 nm), 5th (211 nm), and, in particular, 6th (176 nm) harmonics of the Nd:glass laser is experimentally demonstrated through non-collinearly phase-matched four-wave difference-frequency mixing in fused silica, CaF2 and MgF2 crystals.

Highly efficient third harmonic generation by means of four-wave difference-frequency mixing in fused silica

We report on highly efcient third harmonic generation of 1-ps laser pulses in transparent isotropic solid state mediumthrough phase-matched non-collinear four-wave difference-frequency mixing. Third harmonic pulses at 351 nm with 230 „Jenergy and 15% energy conversion efcienc y were generated in 3-mm-thick UV-grade fused silica sample by use of non-collinear phase matching, cylindrical beam focusing geometry, and pulse-front tilting.

High-power fifth-harmonic generation of femtosecond pulses in the vacuum ultraviolet using a Ti:sapphire laser

We demonstrate the generation of fifth-harmonic pulses at 161 nm, with an energy of up to 600 nJ and 160 fs pulse duration from a Ti:sapphire laser at 1 kHz repetition rate by four-wave difference-frequency mixing in argon-filled waveguides. The efficiency is greatly improved by coupling to higher-order transverse modes, as well as by coating the inner surface of the waveguide. A numerical model of the process yields an understanding of the main effects influencing the harmonic generation.

One-photon mass-analyzed threshold ionization spectroscopy of CH2BrI: Extensive bending progression, reduced steric effect, and spin-orbit effect in the cation

One-photon mass-analyzed threshold ionization (MATI) spectrum of CH2BrI was obtained using coherent vacuum-ultraviolet radiation generated by four-wave difference-frequency mixing in Kr. Unlike CH2ClI investigated previously, a very extensive bending (Br-C-I) progression was observed. Vibrational frequencies of CH2BrI+ were measured from the spectra and the vibrational assignments were made by utilizing frequencies calculated by the density-functional-theory (DFT) method using relativistic effective core potentials with and without the spin-orbit terms. A noticeable spin-orbit effect on the vibrational frequencies was observed from the DFT calculations, even though its influence was not so dramatic as in CH2ClI+. A simple explanation based on the bonding characteristics of the molecular orbitals involved in the ionization is presented to account for the above differences between the MATI spectra of CH2BrI and CH2ClI. The 0-0 band of the CH2BrI spectrum could be identified through the use of combined data from calculations and experiments. The adiabatic ionization energy determined from the position of this band was 9.5944+/-0.0006 eV, which was significantly smaller than the vertical ionization energy reported previously.

One-photon mass-analyzed threshold ionization spectroscopy of 1,3,5-trifluorobenzene: the Jahn-Teller effect and vibrational analysis for the molecular cation in the ground electronic state.

One-photon mass-analyzed threshold ionization spectrum of 1,3,5-trifluorobenzene was obtained by using vacuum ultraviolet radiation generated by four-wave difference frequency mixing in Kr. The Jahn-Teller parameters for the e' modes (nu(8)-nu(14)) of 1,3,5-C(6)H(3)F(3)(+) in the ground electronic state needed for spectral analysis were taken from the density functional theory results initially and were upgraded through fits to the experimental results. Excellent agreement was achieved between the experimental and calculated Jahn-Teller energy levels. Assignments of the Jahn-Teller inactive modes were accomplished by referring to the calculated frequencies and the selection rule. The ionization energy of 1,3,5-trifluorobenzene determined from the position of the 0-0 band was 9.6359+/-0.0006 eV.

Vacuum ultraviolet mass-analyzed threshold ionization spectroscopy of hexafluorobenzene: the Jahn-Teller effect and vibrational analysis.

One-photon mass-analyzed threshold ionization (MATI) spectrum of hexafluorobenzene was obtained by using vacuum ultraviolet radiation generated by four-wave difference frequency mixing in Kr. The ionization energy of hexafluorobenzene determined from the position of the 0-0 band was 9.9108+/-0.0006 eV. To aid the spectral analysis, the Jahn-Teller coupling parameters for four e(2g) modes of C(6)F(6) (+) in the ground electronic state were calculated from the topographical data of the potential energy surface obtained at the density functional theory (DFT) level. These were used in the initial calculation of the energies of the Jahn-Teller states and upgraded through the multimode fit to the experimental data. Excellent agreement between the experimental and calculated frequencies was achieved. The vibrations which are not linear Jahn-Teller active were observed and could be assigned by referring to the frequencies obtained at the DFT level.

One-photon mass-analyzed threshold ionization spectroscopy of 2-bromopropene (2-C3H5Br): Analysis of vibration and internal rotation in the cation

Vibrational spectrum of 2-C3H5Br cation in the ground electronic state was obtained by one-photon mass-analyzed threshold ionization (MATI) spectroscopy using coherent vacuum ultraviolet radiation generated by four-wave difference frequency mixing in Kr. From the MATI spectra, ionization energy of 2-C3H5Br to the ionic ground state has been determined to be 9.4377±0.0006 eV. Vibrational assignments have been made by comparing with the vibrational frequencies and Franck-Condon factors calculated at the B3LYP and BP86 levels with the 6-311++G(3df,3pd) basis set. Several low-frequency bands have been assigned to the torsional motion of the methyl group in the cation. Energies of the torsional states and relative transition intensities to these states have been reproduced well by a one-dimensional rotor model. The torsional barrier and internal rotational constant have been determined to be 80.0 and 5.13 cm−1, respectively.

One-Photon Mass-Analyzed Threshold Ionization Spectroscopy of trans- and cis-1-C3H5Br: Ionization Energies and Vibrational Assignments for the Cations

Vibrational spectra of trans- and cis-1-C3H5Br cations in the ground electronic states were obtained by the one-photon mass-analyzed threshold ionization (MATI) spectroscopy using the coherent vacuum ultraviolet radiation generated by four-wave difference frequency mixing in Kr. From the MATI spectra, ionization energies of trans- and cis-1-C3H5Br to the ionic ground states have been determined to be 9.2693 ± 0.0006 and 9.3140 ± 0.0006 eV, respectively. Almost complete vibrational assignments for the peaks in the MATI spectra were possible by utilizing vibrational frequencies and Franck−Condon factors calculated at the B3LYP and BP86 levels with the 6-311++G(3df,3pd) basis set.

Vibrational Analysis of Vacuum Ultraviolet Mass-Analyzed Threshold Ionization Spectra of Phenylacetylene and Benzonitrile

One-photon MATI spectra of phenylacetylene and benzonitrile were obtained by using vacuum ultraviolet radiation generated by four-wave difference frequency mixing in Kr. Accurate ionization energies of phenylacetylene and benzonitrile were determined from the positions of the 0−0 bands, which are 8.8195 ± 0.0006 and 9.7288 ± 0.0006 eV, respectively. Vibrational frequencies of the molecular ions were determined from the spectra also. Assignments were made by utilizing the symmetry selection rule and frequencies and Franck−Condon factors calculated at the BP86, B3LYP, and B3PW91 density functional theory levels with the 6-311++G(2df,2pd) basis set.

Vacuum ultraviolet mass-analyzed threshold ionization spectroscopy of vinyl bromide: Franck–Condon analysis and vibrational assignment

Vibrational spectrum of vinyl bromide cation in the ground electronic state was obtained by one-photon mass-analyzed threshold ionization (MATI) spectroscopy using coherent vacuum ultraviolet radiation generated by four-wave difference frequency mixing in Kr. From MATI spectrum, ionization energy to the ground state of the cation was determined to be 9.8171±0.0006 eV (79 180±5 cm−1). Almost complete vibrational assignments for the peaks in the MATI spectrum were possible by utilizing vibrational frequencies and Franck–Condon factors calculated at the Becke three parameter Lee–Yang–Parr (B3LYP)/6-311++G(df,pd) level. Franck–Condon analysis for one-photon MATI spectra is especially useful because calculations of only the ground electronic states are involved while that for two-photon MATI spectra requires excited state calculations.

Generation of femtosecond VUV pulses and their application to time resolved spectroscopy in the gas phase

We describe a laser system capable of producing tunable femtosecond VUV pulses around 100 nm by two-photon near-resonant four-wave difference-frequency mixing in argon. Two colour pump-probe ionization experiments allow the characterization of the VUV pulses as well as time-resolved studies of highly excited molecular systems (CS 2 , toluene, HDO) in a supersonic jet.