Attosecond Transient Absorption Spectroscopy of Diatomic Molecules at 400 eV

Intense ultrashort-pulse sources in long wavelength is crucial to extend the spectral range of high harmonic pulses from the extreme ultraviolet (EUV) to soft X-ray region. Optical parametric chirped pulse amplification (OPCPA) allows to produce intense IR pulses with a few-cycle duration with stable carrier-envelope phases (CEPs), which opens the route for attosecond transient absorption spectroscopy (TAS). We developed a CEP-stable ultrafast IR source (λ=1.6 μm, 1.5 mJ, 10 fs at 1 kHz) that used BiB 3 O 4 -based OPCPAs pumped by a Ti:sapphire chirped pulse amplification laser. Because of the extremely broad phase matching condition near the degeneracy, nearly one-octave parametric gain bandwidth is achieved. Using this source and a high-pressure gas cell filled with Ne or He gases, isolated attosecond pulses are successfully produced that almost cover the water window. We applied this soft X-ray source to trace field-driven dynamics of diatomic molecules, NO, via attosecond TAS at nitrogen K edge around 400 eV. Due to tunnel ionization of NO molecules, we observed the emergence and step-like increase of the 1s-2π absorption line of NO + ions. This line also showed a periodic shift due to molecular vibration. We also observed a signature of field-induced molecular alignment of NO and NO + . These results demonstrate the general capability of attosecond soft-X-ray TAS to trace various degrees of quantum dynamics (electronic, vibrational, and rotational) of different energy and time scales.