Laser-pulse requirements for coherent and mode-selective excitation in the quasicontinuum of polyatomic molecules

We show that the theory of nuclear magnetic relaxation applies with equal validity to intramolecular relaxation in the quasicontinuum of polyatomic molecules. This theory permits an analysis of the laser-pulse requirements for vibrational-mode selectivity and coherent multiphoton excitation. The laser Rabi precession frequency, $\mu \mathcal{E}/\mathit{ћ}$, should exceed 1/T₁ or 1/T₂, the intramolecular relaxation rates in the quasicontinuum. In practice this requires either an ultra-fast optical-pulse risetime or a slower pulse that is tuned far to the red side of the transition.