Mitsuhiko Kono

Pulsed injection-seeded optical parametric oscillator with low frequency chirp for high-resolution spectroscopy

An injection-seeded optical parametric oscillator (OPO), based on periodically poled KTiOPO4 and pumped by a frequency-doubled, nanosecond-pulsed Nd:YAG laser, generates continuously tunable, single-longitudinal-mode, pulsed output at approximately 842 nm for high-resolution spectroscopy. Optical-heterodyne measurements show that the OPO frequency chirp increases linearly with detuning from the free-running (unseeded) OPO frequency and can be maintained as low as 10 MHz. Other factors affecting chirp are identified.

Transition from single-mode to multimode operation of an injection-seeded pulsed optical parametric oscillator

Optical-heterodyne measurements are made on ~842-nm signal output of an injection-seeded optical parametric oscillator (OPO) based on periodically poled KTiOPO4 pumped at 532 nm by long (~27-ns) pulses from a Nd:YAG laser. At low pump energies (</= 2.5 times the free-running threshold), the narrowband tunable OPO output is single-longitudinal-mode (SLM) and frequency chirp can be <10 MHz, much less than the transform-limited optical bandwidth (~17.5 MHz). We explore the transition from SLM operation to multimode operation as pump energy or phase mismatch are increased, causing unseeded cavity modes to build up later in the pulse.

Control of frequency chirp in nanosecond-pulsed laser spectroscopy. 3. Spectrotemporal dynamics of an injection-seeded optical parametric oscillator

Complicated spectrotemporal processes are associated with the generation of signal and idler output pulses on a nanosecond timescale in an injection-seeded optical parametric oscillator (OPO). The mechanisms of such spectrotemporal dynamics are revealed by numerical simulation, including innovative modeling of instantaneous-frequency profiles and frequency chirp. These simulations are in satisfactory agreement with optical-heterodyne measurements of output from a nanosecond-pulsed OPO system that is based on periodically poled KTiOPO4, pumped at 532 nm by a Nd:YAG laser and injection-seeded at a signal wavelength of ∼842 nm. Frequency chirp in narrowband signal output pulses from such an OPO system has previously been observed to depend on phase mismatch between the pump, signal, and idler waves, and also on the pump-pulse energy. Our simulations accurately predict this behavior and yield realistic estimates of the frequency chirp, optical bandwidth, and spectral purity of the signal output pulse as it evolves, including effects that are not readily observed directly. This approach provides insight into instrumental conditions that facilitate continuously tunable, single-longitudinal-mode operation of such a pulsed OPO system, with optical bandwidth as close as possible to the Fourier-transform limit.

Heterodyne-assisted pulsed spectroscopy with a nearly Fourier-transform limited, injection-seeded optical parametric oscillator

Narrowband pulsed 822 nm signal radiation from an injection-seeded optical parametric oscillator (OPO) system is used to record fluorescence-detected sub-Doppler two-photon excitation (TPE) spectra of atomic cesium. An optical-heterodyne technique is used to monitor the frequency chirp as well as the fluctuating central frequency of successive OPO output pulses, thereby providing a novel way to record sub-Doppler TPE spectra. The measured TPE linewidth approaches the ultimate limit imposed by the Fourier transform of the pulses temporal profile, demonstrating the utility of this system for pulsed laser spectroscopy applications that require the highest possible resolution.

Near-UV photodissociation dynamics of formic acid

H (Rydberg) atom photofragment translational spectroscopy has been used to study the photodissociation dynamics of jet-cooled formic acid molecules following excitation to their first excited singlet (S1) state at numerous wavelengths in the range 216–241 nm. Analysis of the resulting H-atom time-of-flight spectra indicates contributions from three H-atom formation channels, which we identify as the primary C–H and O–H bond fission processes and the secondary photolysis of HCO() fragments resulting from primary C–O bond fission. It also allows determination of the bond dissociation energies: D0(H–CO2H)≈30000 cm-1 and D0(HCOO–H)=39080±100 cm-1. The former bond fission is deduced to occur after intersystem crossing to the neighbouring a3A″ state, and to involve passage over (or tunnelling through) a barrier in the C–H dissociation coordinate on the triplet potential-energy surface. O–H bond fission, in contrast, is shown to occur predominantly on the S1 surface but it, too, must overcome an activation barrier, the magnitude of which we can estimate at ca. 5400 cm-1, measured relative to the asymptotic products H+HCOO(and/or A). The latter assignment affords a refined value for the 0 K heat of formation of the formyloxyl radical: ΔfH0o (HCOO)=-119.5±3 kJ mol-1.

Near-UV photolysis of methylamine studied by H-atom photofragment translational spectroscopy

H(D) atom photofragment translational spectroscopy has been used in a detailed study of the near-UV photolysis of methylamine and its deuteriated analogues at a number of wavelengths in the range 203.0–236.2 nm. Analysis of the total kinetic energy release spectra so obtained serves to reinforce recent suggestions that at least two dissociation pathways lead to H(D) atom fragments. One, ‘direct’, route involves N—H bond extension on the A state surface, transfer to the ground-state surface via the conical intersection in the N—H exit channel and production of CH3NH fragments carrying substantial internal excitation, behaviour very reminiscent of that exhibited by ammonia following photoexcitation to its corresponding A excited state. Determination of the kinetic energies of the fastest H atoms formed at various of the longer excitation wavelengths yields an improved value for the N—H bond strength in methylamine: D0(H—NHCH3)= 34 550 ± 200 cm–1. A second fragmentation channel yields H(D) atoms with a kinetic energy distribution peaking much closer to zero. Many of these are presumed to arise from the unimolecular decay of highly internally excited ground-state methylamine molecules, themselves formed via internal conversion from the initially prepared A state. RRKM calculations assuming complete energy randomisation in the CH3NH#2 species prior to dissociation suggest, respectively, major and minor contributions to the total H atom yield from channels leading to the products H + CH2NH2 and H + CH3NH, but the observed variations in total H(D) atom signal strength upon isotopic substitution indicate a dominant role for the N—H(D) fission process. Analyses of the measured total kinetic energy release spectra suggest that another component of the ‘direct’ dissociation pathway, yielding electronically excited CH3NH(A) fragments, supplements the slow H atom yield observed at the shortest excitation wavelengths.

Ultraviolet photodissociation dynamics of formyl fluoride Part 2 Energy disposal in the H + FCO product channel

The technique of H (Rydberg) atom photofragment translational spectroscopy has been used to investigate the total kinetic energy release (TKER) into the H] FCO fragments resulting from near UV photolysis of jet-cooled formyl Nuoride, HC(O)F, molecules at numerous wavelengths in the range 248.2E218.4 nm. Analysis of the TKER spectra yield a precise value for the CwH bond dissociation energy cm~1, and allow an estimate of the height of the energy barrier in D0[HEC(O)F]\ 34 950^ 20 the alternative, and previously unobserved, CwF bond Ðssion channel. Photolysis at the longest wavelengths within this range results in products carrying only modest rotational and vibrational excitation (the former concentrated in the form of FCO(X3 ) a-axis rotation) ; the bulk of the available energy Mi.e., appears as product translation. The rotational Ephot[ D0[HEC(O)F]N energy disposal is deduced to be almost invariant to excitation wavelength, but the extent of product vibration is found to increase near linearly with increasing These observations are all explicable in terms of a fragmentation mechanism in which phot . the photo-excited molecules undergo intersystem crossing (ISC) to the neighbouring a8 3AA surface and then evolve HC(O)F(A3 ) over (or through) an energy barrier in the HEC(O)F dissociation coordinate. A simple impact parameter model suffices to show that the rotational energy disposal is determined largely by the geometry and the forces acting as the molecule traverses this barrier region. The observed energy partitioning between FCO vibration and product recoil is explained in terms of a statistical fragmentation process occurring above a potential-energy barrier. The available energy, is viewed in terms of two energy avl , reservoirs : the Ðrst of these, corresponding to the exit channel barrier itself, is released impulsively (mainly into product translation), whilst the other, containing the remainder of is partitioned statistically amongst the various energetically avl , accessible vibrational states of the FCO fragment. H atom tunnelling is explicitly incorporated in the model and serves to blur the sub-division between these two energy reservoirs. Its inclusion allows good replication of all measured TKER spectra within the range of photolysis wavelengths investigated.

Near ultraviolet photolysis of HFCO: the H + FCO channel

The technique of H (Rydberg) atom photofragment translational spectroscopy has been used to study the process HFCO(S1)→H+FCO(X), near its appearance threshold, at excitation wavelengths ca. 247 nm. Analyses of the resulting total kinetic energy release spectra lead to an accurate determination of the C–H bond strength: D0(H–FCO)=34950±20 cm−1. The resulting FCO fragments are observed to be formed with little internal energy, distributed mainly in the form of a-axis rotation. Fragmentation is shown to involve S1–T1 intersystem crossing, followed by rapid passage along the minimum energy path to the eventual H+FCO products. This minimum energy path involves passage over (or H atom tunnelling through) a saddle point, the height of which is ⩾4740 cm−1 above the dissociation asymptote. The observed propensity for a-axis rotation in the FCO product reflects changes in the parent geometry as it evolves along the C–H dissociation coordinate on the T1 surface; past the saddle point, the barrier energy is released...

Effects of clustering of rare-gas atoms on the rate of S1T2 intersystem crossing for 9-methylanthracene

Abstract Laser induced fluorescence excitation spectra for 9-methylanthracene (MEA) in a supersonic free jet were measured as a function of the stagnation pressure of Ar, Kr and Xe, used as carrier gas. The 0-0 excitation bands for complexes of type MEA · Ar n were observed successively corresponding to an increase in coordination number n with increasing stagnation pressure. On the other hand, excitation bands associated with small clusters of Kr and Xe were scarcely observed, although strong excitation bands associated with larger clusters were observed at largely red-shifted wavelengths. These observations combined with the measurement of fluorescence lifetimes were interpreted in terms of two effects of the clustering of rare-gas atoms on the intersystem crossing rate; energy shift of S 1 relative to T 2 and heavy atom effect.

Ultraviolet Photolysis of Formyl Fluoride: the F + HCO Product Channel

The time-of-flight spectrum of the H atoms resulting from photodissociation of gas phase HFCO molecules at 243.12 nm indicates a role for secondary photolysis of HCO() fragments arising via the F+HCO() dissociation channel. Analysis of this spectrum, and of earlier photofragment translational spectroscopy results obtained at a number of neighbouring wavelengths in the range 218.4–248.2 nm, allow estimation of an upper limit for the C–F bond dissociation energy: D0(F–CHO)⩽482 kJ mol-1. HCO() fragments are deduced to be amongst the primary products of HFCO photolysis at all wavelengths λ⩽248.2 nm, indicating that any energy barrier in the F–C bond fission channel [measured relative to the asymptotic products F(2P)+HCO()] must be small. This observation is considered in the light of available knowledge regarding the potential energy surfaces for the ground (1A′) and first excited singlet (A1A″) and triplet (a3A″) states of HFCO; the available evidence all points to radiationless transfer and subsequent dissociation on the triplet surface as the mechanism for the deduced F–C bond fission.