Attosecond Time-Resolved Autoionization of Argon

Autoionization of argon atoms was studied experimentally by transient absorption spectroscopy with isolated attosecond pulses. The peak position, intensity, linewidth, and shape of the $3s3p^{6}np$ ${}^{1}P$ Fano resonance series (26.6–29.2 eV) were modified by intense few-cycle near infrared laser pulses, while the delay between the attosecond pulse and the laser pulse was changed by a few femtoseconds. Numerical simulations revealed that the experimentally observed splitting of the $3s3p^{6}4p$ ${}^{1}P$ line is caused by the coupling between two short-lived highly excited states in the strong laser field.

Figure 1

(a) Energy diagram of the $3s3p^{6}np$ ${}^{1}P$ autoionizing states in argon. The continuum spectrum of the attosecond pulse covers the ${}^{1}P$ series of states. (b) Attosecond transient absorption experimental setup. BS: beam splitter; QP1, BW, QP2, BBO: GDOG optics; GC1: HHG gas cell; F: aluminum filter; TM: toroidal mirror; GC2: absorption gas cell; L: lens; HM: hole mirror; SM: spherical mirror; TG: transmission grating. (c) Transmitted XUV spectrum indicating argon $3s3p^{6}np$ ${}^{1}P$ autoionizing states. The spectrometer resolution was 50 meV.

Figure 2

Transmitted attosecond XUV spectra of argon in a strong NIR laser field with a peak intensity of (a) $5\times {10}^{11}\mathrm{W}/{\mathrm{cm}}^2$ and (b) ${10}^{12}\mathrm{W}/{\mathrm{cm}}^2$. Negative delays correspond to the attosecond pulse arriving on the target before the NIR laser pulse. The resonance peaks are shifted, broadened, and weakened when the two pulses overlap. (c),(d) Transmitted signal (solid) near the $3s3p^{6}4p$ and $3s3p^{6}5p$ states for $5\times {10}^{11}\mathrm{W}/{\mathrm{cm}}^2$ and ${10}^{12}\mathrm{W}/{\mathrm{cm}}^2$, respectively, and calculated exponential decay convoluted with 4.5 fs Gaussian for best fit (dashed).

Figure 3

(a) Schematic representation of argon autoionizing states exposed to the strong NIR laser field. The blue arrows indicate the attosecond XUV excitation of the ground state to the $3s3p^{6}np$ ${}^{1}P$ states as well as to the ${\mathrm{Ar}}^{+}$ ($3s^{2}3p^5\epsilon l$) continuum. The red arrows indicate the NIR laser coupling between the autoionizing states and the ${\mathrm{Ar}}^{*+}$ ($3s3p^6\epsilon l$) continuum or to $3s3p^{6}nl$ autoionizing states. The configuration interaction (green arrows) couples all autoionizing states to the ${\mathrm{Ar}}^{+}$ continuum. (b) Autoionization decay modified by NIR laser-induced coupling to the ${\mathrm{Ar}}^{*+}$ ($3s3p^6\epsilon l$) continuum. Ionization by the NIR field truncates the autoionization decay, resulting in a shorter lifetime and a broader, shifted resonance peak. (c) Autoionization decay modified by NIR laser-induced coupling to $3s3p^{6}nl$ autoionizing states. Rabi oscillation between the two states results in ac Stark-like splitting [9, 10].

Figure 4

Simulated dipole radiation spectrum of laser-induced coupling of the $3s3p^{6}4p$ and $3s3p^{6}4d$ autoionizing states. The XUV laser had a pulse duration of 140 as and intensity of ${10}^{10}\mathrm{W}/{\mathrm{cm}}^2$. The NIR laser had a pulse duration of 8 fs and intensity of (a) $5\times {10}^{11}\mathrm{W}/{\mathrm{cm}}^2$ and (b) ${10}^{12}\mathrm{W}/{\mathrm{cm}}^2$.