Femtosecond gas-phase molecular dynamics: Pump-probe and four-wave mixing experiments

Time-resolved rotational anisotropy measurements can be used to obtain rotational constants of molecules or information about the alignment and rotational energy in chemical reactions. For some of the most common experimental configurations, the well-known expression for obtaining the rotational anisotropy is not applicable; unidirectional signal detection measurements can overestimate the parallel or perpendicular components of the signal. New formulations that take into account different unidirectional detection schemes and f number of the collection optics are given and demonstrated with femtosecond time-resolved anisotropy measurements on iodine vapor. Fits to the calculated anisotropy are shown to provide quantitatively accurate results. In addition, nonlinear saturation effects in ultrafast rotational anisotropy measurements are observed as a function of increased pump laser intensity (ranging over three orders of magnitude). These effects range from a mild reduction in overall anisotropy to the loss of anisotropy at time zero and the appearance of additional photochemical processes. At the highest intensities, the rotational anisotropy measurements show an unusual initial dip followed by a rise near time zero that is due to excitation of a weaker perpendicular state. Experimental results on molecular iodine, chosen as a model system, fit with the conventional anisotropy formalism result in erroneous rotational populations. Incorporating the observation of the perpendicular state into a nonlinear rotational anisotropy model yields accurate rotational populations from measurements with saturated transitions.; Time-resolved transient grating techniques (TG) arising from four-wave mixing (FWM) processes are explored for the study of molecular dynamics in gas-phase systems ranging from single atoms to large polyatomic molecules with nonresonant pulses. Atomic species Ar and Xe show a peak only at zero time delay. For diatomic O₂ and N₂ and linear triatomic CS₂ molecules, the TG signals exhibit ground state rotational recurrences that can be analyzed to obtain accurate rotational constants. Both ground state vibrational and rotational dynamics are observed in the heavier triatomic HgI₂. TG measurements on larger polyatomic molecules (CH₂ Cl₂, CH₂Br₂, benzene, and toluene) show rotational dephasing. A theoretical formalism is developed and used successfully to interpret and simulate the experimental transients. Four-wave mixing experiments with resonant excitation allow us to select between measurements that monitor wave packet dynamics, i.e. populations in the ground or excited states, or coherences between the two electronic states. These cases are explored with the X → B transition in I₂. Control of the population transfer between the ground and excited states is reported using three-pulse four-wave mixing. The inherent vibrational dynamics of the system are utilized in timing the pulse sequence that controls the excitation process. A slight alteration in the pulse sequence timing causes a change in the observed signal from coherent vibration in the ground state to coherent vibration in the excited state.