Femtosecond pump–probe 2D optical Kerr effect spectroscopy of molecular hydrogen crystals

Femtosecond pump–probe 2D (wavelength and delay) optical Kerr effect (OKE) spectroscopy is used to measure laser induced birefringence in parahydrogen and normal hydrogen crystals at 4.2 K. At the probe center wavelength the OKE signal reveals underdamped sinusoidal modulations at frequencies corresponding to the Raman-active transverse optical phonon mode of solid parahydrogen (36.8 cm−1) and normal hydrogen (38.5 cm−1), respectively. For parahydrogen crystals, at small shifts away from the probe center wavelength the OKE signal displays high frequency oscillations (355 cm−1) which correspond to J = 2 rotons with a 94 fs time period.