Ultrafast studies of electronic processes at surfaces using the laser-assisted photoelectric effect with long-wavelength dressing light

We show that ultrafast surface science studies using the laser-assisted photoelectric effect can benefit from longer-wavelength infrared dressing beams. We compare soft-x-ray photoemission from a Pt(111) surface dressed by 1300 and 780 nm light. Using 1300 nm light, the amplitude of the laser-assisted photoelectric signal is enhanced sevenfold compared with 780 nm. This allows lower dressing laser intensity to be used, which dramatically suppresses undesirable processes such as above-threshold photoemission, desorption, and distortion of the photoemission spectrum due to space charge. This work enables ultrafast studies of surface-adsorbate systems and attosecond electron dynamics over a wider energy range.

Figure 1

(Color online) Experimental setup for observing the laser-assisted photoelectric effect from a Pt surface.

Figure 2

(Color online) Laser-assisted photoelectric effect with (a) 780 and (b) 1300 nm dressing pulses. Both spectra are measured at zero time delay between the xuv and ir pulses. In (a) the 780 nm pulses with peak intensity $I_{780\text{nm}}=1.4\times {10}^{11}\text{W}/{\text{cm}}^2$ generate sidebands $A_{1\left(780\text{nm}\right)}=0.13\pm 0.01$, while in (b) the 1300 nm pulses with peak intensity $I_{1300\text{nm}}=2.4\times {10}^{10}\text{W}/{\text{cm}}^2$ generate sidebands $A_{1\left(1300\text{nm}\right)}=0.18\pm 0.01$.

Figure 3

(Color online) Multiphoton photoemission spectra using 780 nm (upper red line) and 1300 nm (lower black line) pulses alone. At the intensities of $I_{780\text{nm}}\approx {10}^{11}\text{W}/{\text{cm}}^2$ required for 780 nm pulses to obtain observable LAPE signal, a large ATP photoelectron flux is observed in the energy range up to 23 eV. In contrast, using 1300 nm pulses, the intensity of $I_{1300\text{nm}}\approx {10}^{10}\text{W}/{\text{cm}}^2$ required for good LAPE signals produces ATP photoelectrons with kinetic energies less than 7 eV.

Figure 4

(Color online) Comparison of space-charge effects around the Fermi edge at $-100\text{fs}$ time delay between the xuv and ir pulses. For 780 nm pulses, the large number of low energy photoelectrons generated through ATP causes a shift of the entire photoemission spectrum around the Fermi edge. This effect is greatly suppressed with 1300 nm pulses, because of the smaller number of ATP photoelectrons.