Our experiments on focusing of ultraviolet $\sim 250\text{−}\mathrm{nm}, \sim 100\text{−}\mathrm{fs}$, 0.2-mJ pulses into a cuvette filled with diatomic air or monatomic argon show similar spectral transformations regardless of the gas if the pressure is the same. The spectra experience broadening and the long-wavelength shift, which increases monotonically with the pressure and reaches 2 nm at 1 atm. This long-wavelength shift cannot be explained by instantaneous Kerr nonlinear response and multiphoton ionization as two additive contributions to the nonlinear refractive index. The simulations based on the 1D time-dependent Schrödinger equation reveal that the cubic nonlinearity is delayed by $\sim 100$ as with regard to the driving field. This intraperiod delay comparable with the $\sim 800\text{−}\mathrm{as}$ field cycle shifts the polarization spectrum to longer wavelengths and can be obtained solely from the quantum-mechanical approach, which we uniquely applied to the description of ultraviolet femtosecond filamentation.

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• Kerr effect
• Multiphoton or tunneling ionization & excitation
• Nonlinear optics