Knowledge Bank

Using the extraordinary high spectral resolution obtainable in nonlinear laser absorption, precision Stark spectra of $CH4$ have been observed in the infrared. Two experiments involving the nonlinear absorption of a tunable 3.39$\mu$ He-Ne laser by the E coriolis component of P(7) in the $\nu 2$-band of $CH4$ are discussed. In one, a single Lamb-dip resonance of $\sim$ 100 kHz width is observed for this transition. Application of a Stark field causes this resonance to split into 13 equally spaced and completely resolved components, in agreement with the assignment and the existence of a small dipole moment in the excited vibrational state. In a second experiment, a Stark-tuned optical-optical double resonance effect yields a precise measurement of this dipole moment as $\mu =0.0200\pm 0.0001$ Debye.