Harmonic Generation from Neutral Manganese Atoms in the Vicinity of the Giant Autoionization Resonance

High harmonics from laser-ablated plumes are mostly generated from ionic species. We demonstrate that with ultrashort infrared ($\sim 1.82\mu \mathrm{m}$) driving lasers, high harmonics from laser-ablated manganese are predominantly generated from neutral atoms, a transition metal atom with an ionization potential of 7.4 eV. Our results open the possibility to advance laser-ablation technique to study the dynamics of neutral atoms of low ionization potential. Moreover, as manganese contains giant autoionizing resonance, intense and broadband high harmonics have been demonstrated from this resonance at energies from 49 to 53 eV. This opens the possibility to generate intense attosecond pulses directly from the giant resonances, as well as to study these resonances using high-harmonic spectroscopy.

Figure 1

Schematic diagram of the experimental setup used for high-harmonic generation.

Figure 2

(a) Harmonic spectra observed from manganese plumes with pulse durations of $\sim 30$ (blue spectra) and 50 fs (black spectra), wavelength of $1.82\mu \mathrm{m}$, and laser intensities of $\sim 4.0\times {10}^{14}\mathrm{W}/{\mathrm{cm}}^2$ and $\sim 5\times {10}^{14}\mathrm{W}/{\mathrm{cm}}^2$, respectively. It is revealed that with the 50 fs pulses, high harmonics are generated from neutral manganese. However, harmonics from GAR are not observed with the 50 fs pulses. The high harmonics from GAR appear for a laser with a pulse duration of 30 fs. (b) The high-harmonic spectra recorded with a few-cycle laser at an intensity of $\sim 4.0\times {10}^{14}\mathrm{W}/{\mathrm{cm}}^2$. In these spectra, a broadband continuum is observed. The high-harmonic intensity is high at energies from 49 to 53 eV. This intensity enhancement indicates that these harmonics are generated from the GAR of neutral manganese.

Figure 3

Calculated high-harmonic spectra from manganese with fields of pulse durations 45 (black spectrum), 30 (red spectrum), and 12 fs (blue spectrum), and a wavelength centered at $1.82\mu \mathrm{m}$. These spectra are simulated with a laser intensity of $1\times {10}^{14}\mathrm{W}/{\mathrm{cm}}^2$ as the upper limit of the intensity averaging.