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  • 1
    ISSN: 1432-0649
    Keywords: PACS: 82.40.Js; 82.50.Gw
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract. The photodissociation dynamics of HNO3 in the electronic S3 (2 1 A ′ ) state leading to the fragments OH and NO2 was investigated in real time. HNO3 was prepared either in a fluorescence cell at room temperature (LIF probing of OH) or rotationally cold in a molecular beam (probing of NO2 by three-photon ionization). A 2 1 A ′ lifetime of 60–80 fs could be obtained from the experimental results, indicating essentially barrierless dissociation. In addition, secondary dissociation of internally excited nascent fragments NO2  * leading to products NO(X 2 Π) and O(3 P) with a characteristic dissociation time of 2.3 ps was observed. This time is surprisingly long when compared with dissociation lifetimes of NO2 from the literature, obtained after direct photoexcitation. The discrepancy is explained by differences in the preparation conditions of the dissociative state of NO2.
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  • 2
    ISSN: 1434-6079
    Keywords: PACS. 52.38.Ph X-ray, γ-ray and particle generation – 52.50.Jm Plasma production and heating by laser beams (laser-foil, laser-cluster, etc.) – 36.40.Vz Optical properties of clusters
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract: Using 50 fs ( ∼ 2×1018 W/cm2) and 2 ps ( ∼ 5×1016 W/cm2) pulses from a Ti:Sa multi-TW laser at 800 nm wavelength large Xe-clusters ( 105...106 atoms per cluster) have been excited. Absolute yield measurements of EUV-emission in a wavelength range between 10 nm and 15 nm in combination with cluster target variation were carried out. The ps-laser pulse has resulted in about 30% enhanced and spatially more uniform EUV-emission compared to fs-laser excitation. Circularly polarized laser light instead of linear polarization results in enhanced emission which is probably caused by electrons gaining higher energies by the polarization dependent optical field ionization process. An absolute emission efficiency at 13.4 nm of up to 0.8% in 2π sr and 2.2% bandwidth has been obtained.
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  • 3
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The energetics and ultrafast dynamics in the H atom transfer configuration of ammonia dimer and trimer clusters have been studied. The clusters are first excited to the electronic A˜ state with a 208 nm femtosecond laser pump pulse. This state is allowed to relax for about 1 ps during which the H-transfer state is formed which is then electronically excited by a time-delayed infrared control pulse at 832 nm and finally ionized with a third femtosecond probe pulse at 416 nm. We have also performed complementary theoretical studies elucidating the experimental findings. For the dimer in the excited NH4(3p)(centered ellipsis)NH2(X˜) state the time-dependent ion signals reveal an isotope-independent short lifetime of about τ6=(130±60) fs which can be explained by a curve crossing with the repulsive NH4(3s)(centered ellipsis)NH2(A˜) state, whereas the trimer signal persists on a time scale being more than one order of magnitude longer and exhibits a very large isotope effect. This is interpreted as being due to internal conversion from the excited state NH3NH4(3p)(centered ellipsis)NH2(X˜) back to the NH3NH4(3s)(centered ellipsis)NH2(X˜) ground state. The analysis of the corresponding photoelectron spectra also confirms the transition energies between the electronic states involved, e.g., ΔE[NH4(3s→3p)(centered ellipsis)NH2]=1.5 eV and ΔE[NH3NH4(3s→3p)(centered ellipsis)NH2]=1.2 eV, as determined by our ab initio calculations. © 2002 American Institute of Physics.
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  • 4
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Applying the femtosecond pump–probe technique combined with the photoelectron–photoion coincidence detection we have studied the time-resolved photoelectron spectra of CF2I2 and its fragments after excitation with 4.65 eV photons. The time-dependent photoion signals reflect the complete dissociation of the CF2I2 molecules with a time constant of (100±30) fs which is preceded by an ultrafast relaxation process with (30±10) fs. The analysis of the electron spectra reveals that three electronic states with different vibrational energies are populated by one photon excitation during the pump pulse. Furthermore, the number of absorbed pump and probe photons for higher order excitation, the ionization potential of CF2I2 and its binding energies in the ionic state have been determined by the electron spectroscopy. Both the ion signals as well as the electron spectra demonstrate that the observed products CF2, I2, and I are formed by dissociation of the excited CF2I2 molecules, but no CF2I has been detected in all experiments with widely spread laser parameters. Thus, we conclude the concerted reaction mechanism to be the dominant dissociation channel while the sequential decay with the CF2I intermediate is negligible. The measured long-living signals for I2+ are suggested as due to molecular detachment after absorption of two pump photons. The detected electron spectra for I+ at longer delay times reflect the formation of highly excited neutral iodine atoms by absorption of at least three pump photons. © 2000 American Institute of Physics.
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  • 5
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 112 (2000), S. 4460-4464 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Using the pump–probe technique with 130 fs laser pulses near 200 nm and near 266 nm the internal conversion of the pyrazine molecule excited to the S2 state has been studied. The lifetime of the S2 state due to internal conversion to the lower electronic states is τIC(2)=(20±10) fs while the lifetime of the secondarily populated S1 state is τIC(1)=(22±1) ps. The results of femtosecond time-resolved electron spectroscopy directly demonstrate the variation of the electron configuration during the internal conversion: The electron spectrum changes significantly on the fs time scale for pyrazine ions produced by ionization via the S2 state with ππ* character and by ionization of S1 state molecules with nπ* configuration after the internal conversion, respectively. The results obtained confirm theoretical estimations of Domcke and co-workers [J. Chem. Phys. 95, 7806 (1991); J. Phys. Chem. 97, 12466 (1993)] who describe the internal conversion in the pyrazine molecule on the basis of a conical intersection of the corresponding potential energy surfaces. © 2000 American Institute of Physics.
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  • 6
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Applying photoion and coincident photoelectron detection in femtosecond pump–probe experiments, we have studied the change of the fragmentation behavior of ammonia clusters excited by femtosecond (fs) laser pulses at 200 nm to the electronic A˜ state which absorb an additional fs control photon 1–2 ps after the pump photon. Only a few 100 fs after the primary excitation, the (NH3)n clusters are partially transferred to the vibrationally highly excited H-transfer state (NH3)n−2NH4NH2 with a lifetime of a few ps. By irradiating the clusters in this state with control photons of a wavelength in the range of 1200–1400 nm, we were able to excite the clusters resonantly to the next higher electronic state in the H-transfer configuration with a strongly reduced vibrational energy. The excited H-transfer state corresponds to the 3s→3p transition in the NH4 component of the internally hydrogenated clusters. Due to the strong reduction of the vibrational energy after the control photon absorption, the fragmentation probability in the excited H-transfer state and correspondingly in the ionic proton transfer state is drastically reduced. For example, for the ammonia dimer the signal ratio of [(NH3)2+] to [NH4+] has been enlarged by nearly one order of magnitude by the resonant control photon absorption. Whereas the lifetime of the ammonia clusters in the nonexcited H-transfer state is nearly identical for all cluster sizes (2–4 ps) we found distinct lifetimes τ6 for the excited H-transfer state of the dimer and the trimer. For the dimer a lifetime τ6=130±50 fs has been obtained for undeuterated as well as for deuterated ammonia molecules. In contrast, for the trimer the lifetime τ6 is significantly larger and depends on the control wavelength as well as on the isotope composition. © 2001 American Institute of Physics.
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