A comparison of the selective excitation of molecular modes in gas and liquid phase using femtosecond pulse shaping

Abstract

A feedback-controlled optimization in a femtosecond coherent-anti Stokes Raman scattering (CARS) process is applied to selectively excite or suppress vibrational modes in the gas and liquid phase. The optimal control experiments are performed on carbon disulfide and toluene molecules. Here our aim is to understand whether the interaction of the molecules with the surrounding medium affects the optimization process. The CARS excitation was chosen to be not in resonance with an electronic transition in the molecule but to excite different vibrational modes coherently. A pure phase modulation of the Stokes pulse resulted in changes of the ratio of the Raman lines observed in the nonlinear scattering spectrum. This could also be achieved when no temporal shift between the pump and Stokes laser resulted in a simple change of the Raman resonances. The relative intensities of the Raman lines could be changed more effectively in the liquid phase than in the gas phase. The higher density in the condensed matter, which hinders free rotation and makes interactions between molecules an important factor, obviously seems to influence the control mechanism.

Searching for an optimal control in the presence of saddles on the quantum-mechanical observable landscape

“…OCEs have been successfully performed for a wide range of goals, including the control of molecular vibrational [13][14][15][16][17][18][19][20] and electronic states [21][22][23][24][25][26][27][28][29], the generation and coherent manipulation of X-rays [30][31][32][33][34], the control of decoherence processes [35,36], the selective cleavage and formation of chemical bonds [37][38][39][40][41][42][43], the manipulation of energy flow in macromolecular complexes [44][45][46][47], and the control of photoisomerization reactions [48][49][50][51][52]. Optimal control theory (OCT) [7,9,[53][54][55][56] has provided insights into the coherent control of a variety of quantum phenomena, such as electron transfer …”

Searching for quantum optimal controls under severe constraints

“…Successful optimal control experiments (OCEs) have included selective control of molecular vibrational [10][11][12][13][14][15][16][17] and electronic states [18][19][20][21][22][23][24][25][26][27], preservation of quantum coherence [28,29], control of photoisomerization reactions [30][31][32][33][34][35], selective manipulation of chemical bonds [36][37][38][39][40][41][42][43][44], high-harmonic generation and coherent manipulation of the resulting soft X-rays [45][46][47][48][49][50][51], and control of energy flow in biomolecular complexes [52][53][54][55]. Optimal control theory (OCT) [7,9,…”

Recent advances in linear and non‐linear Raman spectroscopy. Part III

“…A feedback-controlled optimisation in a femtosecond CARS process was demonstrated by Scaria et al [73] to selectively excite or suppress vibrational modes in the gas and liquid phases. The optimal control experiments were performed on carbon disulfide and toluene molecules in order to determine whether the interaction of the molecules with the surrounding medium affects the optimisation process.…”