“…The narrow spectral fringes separated by 2.7 THz in the experimental plot are caused by beating with light in the LP11 mode, accidentally excited by imperfect launching conditions (the group velocity walk-off between the LP01 and the LP11 mode is ~4.6 fs/cm at 400 nm, accumulating to 370 fs after 80 cm, i.e., the inverse of 2.7 THz). The best agreement between experiment and theory was obtained for fr,air = 0.75, μrot,N2 = 0.986 and μrot,O2 = 1, in agreement with the values reported in [19] and confirming that the rotational responses of N2 and O2 dominate in the Raman response of air. Of these, the S(8)-transition of N2 (period 439 fs, frequency shift 2.28 THz) and the S(11)-transition of O2 (period 463 fs, frequency shift 2.16 THz) are long enough to be impulsively excited by the 54-fs pump pulses.…”