التفاصيل البيبلوغرافية
| العنوان: |
Cooling dynamics of an optically excited molecular probe in solution from femtosecond broadband transient absorption spectroscopy. |
| المؤلفون: |
Kovalenko, S. A., Schanz, R., Hennig, H., Ernsting, N. P. |
| المصدر: |
Journal of Chemical Physics; 8/15/2001, Vol. 115 Issue 7, p3256, 18p, 4 Charts, 19 Graphs |
| مصطلحات موضوعية: |
MOLECULAR probes, COOLING |
| مستخلص: |
The cooling of p-nitroaniline (PNA), dimethylamino-p-nitroaniline (DPNA) and trans-stilbene (t-stilbene) in solution is studied experimentally and theoretically. Using the pump–supercontinuum probe (PSCP) technique we observed the complete spectral evolution of hot absorption induced by femtosecond optical pumping. In t-stilbene the hot S[sub 1] state results from S[sub n]→S[sub 1] internal conversion with 50 fs characteristic time. The time constant of intramolecular thermalization or intramolecular vibrational redistribution (IVR) in S[sub 1] is estimated as τ[sub IVR]<100 fs. In PNA and DPNA the hot ground state is prepared by S[sub 1]→S[sub 0] relaxation with characteristic time 0.3–1.0 ps. The initial molecular temperature is 1300 K for PNA and 860 K for t-stilbene. The subsequent cooling dynamics (vibrational cooling) is deduced from the transient spectra by assuming: (i) a Gaussian shape for the hot absorption band, (ii) a linear dependence of its peak frequency ν[sub m] and width square Γ[sup 2] on molecular temperature T. Within this framework we derive analytic expressions for the differential absorption signal ΔOD(T(t),ν). After calibration with stationary absorption spectra in a low temperature range, the solute temperature T(t) may be evaluated from a transient absorption experiment. For highly polar PNA and DPNA, T(t) is well described by a biexponential decay which reflects local heating effects, while for nonpolar t-stilbene the local heating is negligible and the cooling proceeds monoexponentially. To rationalize this behavior, an analytic model is developed, which considers energy flow from the hot solute to a first solvent shell and then to the bulk solvent. Fastest cooling is found for PNA in water: a time constant of 0.64 ps (68%) corresponds to solute–solvent energy transfer while 2.0 ps (32%) characterizes the cooling of the first shell. In aprotic solvents cooling is slower... [ABSTRACT FROM AUTHOR] |
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| قاعدة البيانات: |
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