Toshiaki Munakata

Fluorescence lifetimes of no A2Σ+(ν′= 3 and 4), C2Π(ν′= 0) and D2Σ+(ν′= 0) studied by tunable VUV laser excitation

Abstract The fluorescence lifetimes of NO molecules have been investigated under collision-free conditions in a free jet expansion. Single-photon absorption of the laser-generated, tunable and monochromatic VUV radiation prepared NO molecules in the electronically excited A 2 Σ + , C 2 Π and D 2 Σ + states near the dissociation limit. The lifetimes of the C 2 Π 1 2 , 3 2 (ν′= 0)-B 2 Π 3 2 (ν′= 7) perturbed states showed a significant rotational level dependence which can be explained in terms of the known interactions between the B and C states. The measured lifetimes for the A 2 Σ + (ν′= 3 and 4) states, 163.9±3.5 and 137.1 ±2.5 ns, respectively, showed no rotational level dependence. A new dependence of the electronic transition moment on internuclear distance has been derived for the A 2 Σ + -X 2 Π transition to account for the variation of the radiative lifetimes of the A 2 Σ + (ν′= 0, 1, 2 and 3) states. The lifetimes of the A 2 Σ + (ν′= 4) and D 2 Σ + (ν′= 0) states indicated that these states are not predissociative although they are located above the dissociation limit.

Electronic excited state of NO adsorbed on Cu(111): A two‐photon photoemission study

Two‐photon photoemission was observed from molecularly adsorbed NO on Cu(111) at 110 K. Excitation and photoemission were performed with a laser light of 5 ns duration in the photon energy range from 3.8 to 4.5 eV. The NO‐induced photoemission feature was found to be composed of two components: one was attributed to the sequential two‐photon photoemission from the ground state of adsorbed NO, the other to the one‐photon photoemission from the NO negative‐ion state which was formed by hot electrons in the Cu substrate. The two‐photon photoemission intensity from the ground state NO was resonantly enhanced at the photon energy of 3.98±0.05 eV. The photon energy dependence was attributed to resonance excitation to the excited state of adsorbed NO. Width of the resonance curve was 0.3 eV, suggesting that the lifetime of the excited state is around 2 fs. The negative‐ion state of adsorbed NO was 1.26±0.07 eV above the Fermi level. The energy width of the photoemission feature from the negative‐ion state was ab...

Inhomogeneous electronic structure of copper phthalocyanine film measured with microspot photoemission spectroscopy

We have applied photoemission microspectroscopy to copper phthalocyanine film grown on a polycrystalline copper with a lateral resolution of 0.3μm and an energy resolution of 30meV. The photoemission band due to the highest occupied molecular orbital peaked at the binding energy of either 1.6 or 1.2eV depending on the sample positions, while the work functions were 4.3 and 4.5eV for the respective positions. The band was intense when the binding energy was low. The results demonstrate that a large inhomogeneity exists in the interface electronic structure.

Collision energy dependence of vibrational/rotational distribution of BaBr produced in the crossed beam reaction Ba+CH3Br

The energy disposal in the reaction Ba+CH3Br→BaBr+CH3 was studied as a function of collision energy. The collision energy was controlled in the range from 2.9 and 4.9 kcal/mol by variable acceleration of a supersonic CH3Br beam which intersected a thermal Ba beam. The internal states of the nascent BaBr were probed by means of the laser induced fluorescence (LIF). Systematic simulations of the observed LIF spectra revealed that an increase of collision energy brought about a drastic decrease of vibrational excitation. The vibrational quantum of maximum population shifted from 12 to 1 in accordance with a collision energy increment of only 2 kcal/mol. Surprisal analysis of the vibrational population suggests that the collision energy increment causes a transition between two reaction paths, one of which leads to vibrationally excited BaBr of maximum population at around ν″=12, and the other, to internally cold CH3 and statistically excited BaBr.

Time-resolved photoemission microspectroscopy based on fs-VUV laser light

A novel photoemission microspectrometer based on focused VUV coherent radiation is presented for simultaneous realization of <100 fs time resolution, <30 meV energy resolution, and sub-micrometer lateral resolution. VUV light at 140 nm wavelength (8.9 eV photon energy) is generated by frequency upconversion of a regeneratively amplified Ti:sapphire laser of 100 fs pulse duration. The VUV light is focused on a sample surface by a Schwarzschild objective of 0.29 numerical aperture. Photoelectrons are detected by a time-of-flight electron spectrometer. The width of the Fermi edge feature of cooled (30 K) polycrystalline copper was observed to be narrower than 40 meV. The energy resolution is close to the Fourier-transform-limited bandwidth of the laser light. Two-photon photoemission with 420 nm light revealed that the lateral resolution is very close to the diffraction-limited spot diameter of 0.9 μm. The microspectrometer is also equipped with a photoemission electron microscope for time-resolved sub-100 nm imaging.

Direct observation of the B″ 3Πu–X 3Σ−g transition of diatomic sulfur in a supersonic free jet

The laser‐induced fluorescence (LIF) spectra of diatomic sulfur are studied in a supersonic free jet which is expanded from a high temperature nozzle source of 800 K. The lifetime separation method is advantageously combined with the LIF detection in the supersonic free jet. The weak B″ 3Πu–X 3Σ−g band is thereby observed successfully in good distinction among the strong B 3Σ−u–X 3Σ−g band system. The rotational constants and term values of the vibronic levels v′=3–18 B″ 3Πu (F1,F2) are evaluated by a rotational analysis. An inspection of the intensity distribution among the observed spectral lines indicates that the B″ 3Πu–X 3Σ−g bands borrow intensity from B″ 3Σ−u–X 3Σ−g bands through homogeneous perturbations. The anomalous behavior of molecular constants, including the irregular vibronic dependence of the spin‐splitting constant of B 3Σ−u, is interpreted in terms of the perturbation from the B″ 3∏u state.

Intermolecular and interlayer interactions in copper phthalocyanine films as measured with microspot photoemission spectroscopy

The authors have applied photoemission microspectroscopy to copper phthalocyanine films grown on a graphite surface with a lateral resolution of 0.3μm and an energy resolution of 30meV. The photoemission peak due to the highest occupied molecular orbital was found to be at binding energies of 1.13, 1.23, 1.38, and 1.5eV, depending on film thickness. From the thickness and light-polarization dependence, the peaks were assigned to originate from isolated molecules, the first layer, the second layer, and multilayer, respectively. They demonstrate the capability of photoemission microspectroscopy to resolve electronic states modified by fine differences of molecular environments.

Bimodal vibrational distribution of BaBr in the reaction Ba+CF3Br

Nascent vibrational distributions of BaBr were studied for the reaction Ba+CF3Br →BaBr+CF3. In a crossed beam configuration of supersonic CF3Br beam and effusive Ba beam, the product state distribution was measured by the laser induced fluorescence as a function of collision energy. The vibrational distribution of BaBr clearly showed a bimodal feature. At low collision energy, BaBr was produced preferentially around ν″=32, while at some higher collision energy, another vibrational component gained intensity at around ν″=15. The average vibrational energy of the higher vibrational component was about 80% of the total available energy, and that of the lower, about 35%. The bimodality of vibrational distribution indicates that the reaction Ba+CF3Br proceeds along two reaction pathways. In one pathway, Ba approaches in the direction of Br–CF3, and forms BaBr in low vibrational states. Whereas in the other, the atom approaches in the CF3–Br direction, and leads to a high vibrational excitation of BaBr. The bra...

Microspot photoemission spectrometer based on FS-VUV radiation

Abstract The lateral resolution of our laser-based microspot photoemission spectrometer [Surf. Sci. 507–510 (2002) 434] has been improved to 0.3 μm by focusing VUV light of 140 nm wavelength (8.9 eV photon energy). The VUV light is generated by frequency tripling a second-harmonic output of a regeneratively amplified titanium:sapphire laser of 250 kHz repetition rate, and is focused on a sample surface by a Schwarzschild objective of 0.29 NA. Time-of-flight (TOF) photoelectron spectra of energy resolution better than 40 meV are accumulated for 0.4 s during scanning the sample position by 0.1 μm/step. Measurement on a Ta-striped Si sample revealed that the lateral resolution is 0.3 μm, which is very close to the diffraction-limited spot diameter. The present microspectrometer is characterized with the simultaneous realization of the diffraction-limited lateral resolution and the high-energy resolution, which is close to the Fourier transform of the pulse width of the laser light (100 fs).

DISPERSION OF AN ADSORPTION-INDUCED ELECTRONIC STATE OF BENZENE/CU(111)

An angle resolved photoemission work for a benzene-adsorbed Cu(111) surface with a photon energy of 5.45 eV has revealed that an adsorption-induced state due to hybridization of benzene π* and copper shows dispersion with electron momentum along the surface. The dispersion is reproduced well by a parabola with an effective electron mass ratio m*/me=0.9. This suggests that the bonding state extends many copper lattice spacings from the adsorbed molecule. The bonding orbital is thought to be the origin of long range interactions of molecules on a metal surface.