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  • American Institute of Physics (AIP)  (7)
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  • 1
    Electronic Resource
    Electronic Resource
    Control of the fragmentation of excited ammonia clusters by femtosecond infrared laser pulses (2001)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 115 (2001), S. 277-284 
    Details
    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.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1377889
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  • 2
    Electronic Resource
    Electronic Resource
    Analysis of the ultrafast photodissociation of electronically excited CF2I2 molecules by femtosecond time-resolved photoelectron spectroscopy (2000)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 113 (2000), S. 1705-1713 
    Details
    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.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.481972
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  • 3
    Electronic Resource
    Electronic Resource
    Ultrafast photodissociation dynamics and energetics of the electronically excited H atom transfer state of the ammonia dimer and trimer (2002)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 116 (2002), S. 1443-1456 
    Details
    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.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1429952
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  • 4
    Electronic Resource
    Electronic Resource
    Electron configuration changes in excited pyrazine molecules analyzed by femtosecond time-resolved photoelectron spectroscopy (2000)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 112 (2000), S. 4460-4464 
    Details
    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.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.481008
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  • 5
    Electronic Resource
    Electronic Resource
    Real time observation of hydrogen transfer: Femtosecond time-resolved photoelectron spectroscopy in the excited ammonia dimer (1999)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 111 (1999), S. 633-642 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The energy flow in ammonia dimers excited to the electronic A˜ state is analyzed by combining the femtosecond pump–probe technique and the photoelectron–photoion coincidence detection. We use ∼140 fs laser pulses (200 nm for excitation and 267 nm for ionization). For the dimer ion the photoelectron spectra change drastically from a rather broad shape ((approximately-greater-than)1 eV) at small delay times between pump and probe pulse to a rather narrow peak (0.25 eV) at some picoseconds. This is explained by the dynamics of an internal H-atom transfer in the electronic A˜ state to an NH4...NH2 configuration. The measured photoelectron energies are consistent with ab initio potential energy surface calculations. The observed picosecond lifetime of the hydrogen-transfer state NH4...NH2 can be understood by a conical intersection with the charge-transfer state NH4+...NH2−. © 1999 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.479343
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  • 6
    Electronic Resource
    Electronic Resource
    Ultrafast internal conversion and photodissociation of molecules excited by femtosecond 155 nm laser pulses (1999)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 111 (1999), S. 6264-6270 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The dynamics of several prototypical molecular systems after excitation with femtosecond laser pulses at 155 nm has been studied in pump–probe experiments. The vacuum ultraviolet (VUV) pump pulses with a pulse width of 350–450 fs were generated by near-resonant four-wave difference frequency mixing in argon. The careful analysis of the time-dependent ion signals has allowed us to determine the lifetime of the excited molecular states down to about 30 fs. The extremely short lifetime of water molecules excited to the repulsive A˜ state has been directly observed for the first time: τD≤20 fs. For molecular oxygen highly excited in the Schumann–Runge band, a decay time of 40±20 fs was obtained. The lifetimes of ethylene and chloroethylenes as well as of benzene and toluene reaching from 40 up to 180 fs are primarily caused by internal conversion. The decay times τD=(1.9±0.1) and τD=(90±20) ps obtained for carbon disulfide and nitric oxide, respectively, are due to predissociation of the VUV excited states. © 1999 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.479932
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  • 7
    Electronic Resource
    Electronic Resource
    Ultrafast predissociation and coherent phenomena in CS2 excited by femtosecond laser pulses at 194–207 nm (1999)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 111 (1999), S. 5338-5343 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The ultrafast predissociation dynamics of CS2 molecules excited to the 1B2(1Σu+) electronic state by femtosecond laser pulses with 6.0–6.4 eV photon energy has been studied in pump–probe experiments. The analysis of the time-dependent ion signals has revealed lifetimes decreasing from 620 fs down to 180 fs for tuning the excitation wavelength from 207 nm to 194 nm. A nearly constant plateau at about 200 nm in the energy dependence of the lifetime reflects the barrier of the transition from the bent to a quasilinear geometry of the excited CS2 molecule. If two vibrational bands of the 1B2(1Σu+) state were excited simultaneously by the femtosecond laser pulses we observed quantum beats with a modulation frequency corresponding to the energy difference of the two modes. Thus, the coherent excitation process is directly visualized despite the ultrafast decay of the excited molecular states due to predissociation. © 1999 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.479793
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