Hiroshi Kohguchi

FEMTOSECOND TIME-RESOLVED PHOTOELECTRON IMAGING ON ULTRAFAST ELECTRONIC DEPHASING IN AN ISOLATED MOLECULE

Ultrafast dephasing in an intermediate case of molecular radiationless transition has been visualized for the first time by femtosecond time-resolved photoelectron imaging. The decay of photoexcited S1(n,π*) state of pyrazine in 100 ps and the corresponding build-up of triplet states were clearly observed.

A crossed molecular beam apparatus using high-resolution ion imaging

A new crossed molecular beam apparatus with a high-resolution ion imaging detector is described. Two pulsed supersonic molecular beams are crossed at right angles in a vacuum of 10−7 Torr. The collision region is irradiated with a tunable laser pulse that ionizes the scattered particles state selectively. The generated ions are accelerated by stacked electrodes in a two-dimensional (2D) space focusing mode that increases the velocity resolution of the apparatus. A cylindrical hexapole deflector is placed in the middle of the time-of-flight mass spectrometer to compensate the center-of-mass velocity of the ions and to direct them to the center of the 2D imaging detector. Real-time image processing of the charge coupled device camera signal eliminates blurring of the image detector. The performance of the apparatus was examined by observing the inelastic scattering of NO+Ar at a collision energy of 66 meV. The observed multiple rainbow peaks clearly demonstrate the high performance of the apparatus.

Photodissociation of nitrous oxide starting from excited bending levels

The photodissociation dynamics of N2O in the wavelength region of 203-205 nm was studied by velocity map ion imaging. A speed resolution of 0.8% was obtained using standard projection imaging and subpixel centroiding calculations. To investigate N2O dissociation starting from the excited bending levels in the ground electronic state, a supersonic molecular beam and an effusive beam were used. The photoabsorption transition probability from the first excited bending level in the wavelength region of 203-205 nm was estimated to be seven times greater than that from the ground vibrational level.

Detection of metastable triplet acetylene produced by intersystem crossing from the excited Ã(1Au) state

Triplet metastable species produced by intersystem crossing from the A(1Au) state of acetylene has been detected by the sensitized phosphorescence method. A sensitized phosphorescence signal was observed from vibronic levels lying lower than the potential energy barrier for dissociation in the a state suggested previously, but was not observed from levels higher than this barrier. The lifetimes of triplet states produced by intersystem crossing from the V3K1(J′=2) and V4K1(J′=2) levels were estimated to be 100 and 80 μs.

He I Ultraviolet Photoelectron Spectroscopy of Benzene and Pyridine in Supersonic Molecular Beams Using Photoelectron Imaging

We performed He I ultraviolet photoelectron spectroscopy (UPS) of jet-cooled aromatic molecules using a newly developed photoelectron imaging (PEI) spectrometer. The PEI spectrometer can measure photoelectron spectra and photoelectron angular distributions at a considerably higher efficiency than a conventional spectrometer that uses a hemispherical energy analyzer. One technical problem with PEI is its relatively high susceptibility to background electrons generated by scattered He I radiation. To reduce this problem, we designed a new electrostatic lens that intercepts background photoelectrons emitted from the repeller plate toward the imaging detector. An energy resolution (ΔE/E) of 0.735% at E = 5.461 eV is demonstrated with He I radiation. The energy resolution is limited by the size of the ionization region. Trajectory calculations indicate that the system is capable of achieving an energy resolution of 0.04% with a laser if the imaging resolution is not limited. Experimental results are presented for jet-cooled benzene and pyridine, and they are compared with results in the literature.

Acceleration of the Reaction OH + CO → H + CO2 by Vibrational Excitation of OH

The collision complex formed from a vibrationally excited reactant undergoes redissociation to the reactant, intramolecular vibrational relaxation (randomization of vibrational energy), or chemical reaction to the products. If attractive interaction between the reactants is large, efficient vibrational relaxation in the complex prevents redissociation to the reactants with the initial vibrational energy, and the complex decomposes to the reactants with low vibrational energy or converts to the products. In this paper, we have studied the branching ratios between the intramolecular vibrational relaxation and chemical reaction of an adduct HO(v)-CO formed from OH(X(2)Π(i)) in different vibrational levels v = 0-4 and CO. OH(v = 0-4) generated in a gaseous mixture of O(3)/H(2)/CO/He irradiated at 266 nm was detected with laser-induced fluorescence (LIF) via the A(2)Σ(+)-X(2)Π(i) transition, and H atoms were probed by the two-photon excited LIF technique. From the kinetic analysis of the time-resolved LIF intensities of OH(v) and H, we have found that the intramolecular vibrational relaxation is mainly governed by a single quantum change, HO(v)-CO → HO(v-1)-CO, followed by redissociation to OH(v-1) and CO. With the vibrational quantum number v, chemical process from the adduct to H + CO(2) is accelerated, and vibrational relaxation is decelerated. The countertrend is elucidated by the competition between chemical reaction and vibrational relaxation in the adduct HOCO.

Super-Resolution Photoelectron Imaging with Real-Time Subpixelation by Field Programmable Gate Array and Its Application to NO and Benzene Photoionization †

We have constructed a photoelectron imaging spectrometer with super-resolution image processing and have applied it to the photoionization of nitric oxide and benzene in molecular beams. A field programmable gate array is employed for real-time subpixel centroiding calculations on hardware, providing 64 megapixel resolution (8192 x 8192 pixels). We examined eight different centroiding algorithms based on the center-of-gravity (COG) and Gaussian fitting (Gauss) methods and have found that the two-dimensional COG (2D-COG) and weighted mean of Gaussian center (w-Gauss) methods have the best performance. The excellent performance of the instrument is demonstrated by visualizing a 25 mum diameter pore structure of an MCP, indicating a spatial resolution of 0.03%. The photoelectron image in one-color (1 + 1) resonance-enhanced multiphoton ionization of nitric oxide using a nanosecond laser provided a photoelectron kinetic energy resolution of 0.2%. This resolution is currently restricted by charged-particle optics. The photoelectron energy and angular distributions in the one-color (1 + 1) resonance-enhanced multiphoton ionization of benzene via 6(1) and 6(1)1(1) vibronic levels in the S(1) state are also presented. The results demonstrate that photoelectron angular anisotropy varies with the photoelectron kinetic energy and the vibronic state of the cation.

Kinetic study of vibrational energy transfer from a wide range of vibrational levels of O2(X(3)Sigma(g)-, v = 6-12) to CF4.

A wide range of vibrational levels of O2(X(3)Sigma(g)(-), v = 6-13) generated in the ultraviolet photolysis of O3 was selectively detected by the laser-induced fluorescence (LIF) technique. The time-resolved LIF-excited B(3)Sigma(u)(-)-X(3)Sigma(g)(-) system in the presence of CF4 has been recorded and analyzed by the integrated profiles method (IPM). The IPM permitted us to determine the rate coefficients k(v)(CF4) for vibrational relaxation of O2(X(3)Sigma(g)(-), v = 6-12) by collisions with CF4. Energy transfer from O2 (v = 6-12) to CF4 is surprisingly efficient compared to that of other polyatomic relaxation partners studied so far. The k(v)(CF4) increases with vibrational quantum number v from [1.5 +/- 0.2(2sigma)] x 10(-12) for v = 6 to [7.3 +/- 1.5(2sigma)] x 10(-11) for v = 12, indicating that the infrared-active nu3 vibrational mode of CF4 mainly governs the energy transfer with O2(X(3)Sigma(g)(-), v = 6-12). The correlation between the rate coefficients and fundamental infrared intensities has been discussed based on a comparison of the efficiency of energy transfer by several collision partners.

Photodissociation dynamics of C3H5I in the near-ultraviolet region

The ultraviolet photodissociation dynamics of allyl iodide (C3H5I) have been studied by ion-imaging at 266 nm and 213 nm. These photolysis wavelengths are located in the two lowest absorption bands in the near-ultraviolet region. The atomic iodine products were detected by [2+1] resonantly enhanced multiphoton ionization spectroscopy. The spectra showed that the branching fraction for the spin-orbit excited ((2)P(1/2)) state was larger than that for the ground ((2)P(3/2)) state at both photolysis wavelengths. The state-resolved scattering images of iodine showed two maxima in the velocity distributions in the (2)P(3/2) state and a single peak in the (2)P(1/2) state. The spin-orbit specificity indicates that the C-I bond cleavage at both absorption bands is governed by the dissociative n(I)σ*(C-I) potential energy surfaces. The nascent internal energy distribution of the allyl radical (C3H5) counter product, which was obtained by the analysis of the state-resolved scattering distributions, showed a marked difference between the photolysis at 266 nm and 213 nm. The generation of the colder C3H5 with the higher translational energy at 266 nm implied the direct photoexcitation to the n(I)σ*(C-I) repulsive surfaces, whereas the internally hot C3H5 at 213 nm was ascribed to the local π(CC)π*(CC) photoinitiation in the allyl framework followed by predissociation to the n(I)σ*(C-I) states.

Unravelling the Electronic State of NO2 Product in Ultrafast Photodissociation of Nitromethane

The primary photochemical reaction of nitromethane (NM) after ππ* excitation is known to be C-N bond cleavage (CH3NO2 + hν → CH3 + NO2). On the other hand, NO2 can be formed in both the ground and excited states, and identification of the electronic state of the NO2 product has been a central subject in the experimental and theoretical studies. Here we present time-resolved photoelectron spectroscopy using vacuum-ultraviolet probe pulses to observe all transient electronic states of NM and the reaction products. The result indicates that ultrafast internal conversion occurs down to S1 and S0 within 24 fs, and the dissociation proceeds on the S1 surface (τdiss ≲ 50 fs), leading to comparable product yields of NO2(A) and NO2(X). The overall dissociation quantum yield within our observation time window (<2 ps) is estimated to be 0.29.