Enhancement of laser-induced molecular alignment by simultaneous photodissociation

From a three-dimensional time-dependent Schr\"odinger equation simulation for alignment of ${\mathrm{Cl}}_{2}$ by short laser pulses, we show that alignment of the ground state $(X{}^{1}{\ensuremath{\Sigma}}_{g}^{+})$ can be increased by photodissociation via a repulsive dissociative state ${(}^{1}{\ensuremath{\Pi}}_{u}).$ Initially, an intense nonresonant laser pulse ($2\ifmmode\times\else\texttimes\fi{}{10}^{13}{\mathrm{W}/\mathrm{c}\mathrm{m}}^{2},$ 1064 nm) with linear polarization aligns the molecule in the direction of the polarization as measured by $〈{\mathrm{cos}}^{2}\ensuremath{\theta}〉,$ which depends on the $J,$ ${M}_{J}$ initial conditions. Then, a second pulse with the same polarization is added in order to make perpendicular resonant transitions to the repulsive state $(1\ifmmode\times\else\texttimes\fi{}{10}^{13}{\mathrm{W}/\mathrm{c}\mathrm{m}}^{2},$ 375 nm). We show as an example for $J=4$ that the second pulse increases the alignment for each initial ${M}_{J}$ condition as measured by $〈{\mathrm{cos}}^{2}\ensuremath{\theta}{〉}_{{J=4,M}_{J}}$ and the average alignment $〈〈{\mathrm{cos}}^{2}\ensuremath{\theta}〉{〉}_{J=4}$ by reducing the proportion of the ${M}_{J}\ensuremath{\ne}0$ states. This improvement in the alignment of the molecule increases the amplitudes of the average rotational recurrences after the laser pulse and reduces the randomness in these recurrences by removing nonaligned ${M}_{J}\ensuremath{\ne}0$ states.

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