Experimental ionization of atomic hydrogen with few-cycle pulses

Abstract

We present the first experimental data on strong-field ionization of atomic hydrogen by few-cycle laser pulses. We obtain quantitative agreement at the 10% level between the data and an ab initio simulation over a wide range of laser intensities and electron energies.The interaction of intense few-cycle infrared laser pulses with matter induces tunneling ionization and subsequent quantum dynamics of freed electrons. Intense few-cycle pulses are difficult to generate and use because of the stringent requirements on dispersion control over a broad bandwidth. However, they offer unparallelled opportunities to reveal and control the electronic dynamics of atoms [1,2] and molecules [3,4] and to generate isolated attosecond pulses in the extreme ultraviolet [5]. The few-cycle regime is particularly challenging for simulations, as intensities approaching 10 15 W/cm 2 can be reached before the ionisation response saturates. At these intensities, a photoelectron driven by intense longwavelength radiation can travel a distance hundreds of times larger than the size of the parent atom and can have energies of many tens of eV, imposing stringent requirements on the simulation grid. Ab initio simulations in this regime can be carried out only for atomic H due to its simple electronic structure.Here we describe an experiment on the interaction of intense few-cycle laser pulses with atomic hydrogen (H), the simplest of all atomic systems and the traditional test case for atomic physics. No data on H has previously been available in this regime of laser interaction. Previous strong-field experiments with atomic H [6,7] used relatively short-wavelength pulses that were many optical cycles in duration with maximum intensities of 10 14 W/cm 2 . Our data show excellent quantitative agreement, at the 10% level, with ab initio simulation over a wide range of electron energies and laser intensities.The experimental apparatus is composed of an atomic H beam interacting with a few-cycle strong-field laser (Fig. 1). The laser used is a commercial Femtolaser 'Femtopower Compact Pro'. Each pulse has energy of 150 µJ and the pulse repetition rate is 1 kHz. The spectral width of the laser is 150 nm at full width half maximum (FWHM) centered at 750 nm. The pulse duration at FWHM of the intensity envelope is 6.3 ± 0.2 fs at the interaction region, or alternatively ∼ 2.5 optical cycles. An off-axis parabolic mirror of 750 mm focal length is used to focus the beam to a spot size of 47 µm 1/e 2 radius. The laser carrier-envelope phase was not stabilized in these experiments.The atomic H beam is created via collisional dissociation in a radio frequency (RF) discharge powered by a helical resonator [8]. An RF signal at a frequency of 75 MHz and power of 8 W is applied to the resonator and the dissociation efficiency is determined via emission spectroscopy of the discharge. The atomic beam emerging from the discharge is 80 ± 15% H atoms by number with the remainder being undissociated H 2 . The atomic H beam passes through two apertures, producing a...

Precise and Accurate Measurements of Strong-Field Photoionization and a Transferable Laser Intensity Calibration Standard

“…In recent years, our group has used atomic hydrogen (H) to perform quantitatively accurate strong-field measurements that are demonstrably free from systematic errors [10][11][12]. These measurements are performed in the regime of laser pulse durations and peak intensities that are most widely used in attosecond science.…”

“…The experimental apparatus is detailed in [10,11]. It consists of a well-collimated atomic H beam skimmed from the output of an RF discharge dissociator, which intersects the focus of an intense few-cycle laser.…”

“…It consists of a well-collimated atomic H beam skimmed from the output of an RF discharge dissociator, which intersects the focus of an intense few-cycle laser. The flux of our custom-constructed atomic source is several orders of magnitude higher than commercial sources [18], facilitating a high signal level. The laser generates pulses with rapidly varying carrier-envelope phase (CEP) of 5.5 fs duration (measured at full-width at half-maximum of intensity) with a central wavelength of 800 nm.…”

Accurate determination of absolute carrier-envelope phase dependence using photo-ionization

“…ATI of atomic H is perhaps the best match for this condition, but it is certainly not easy to realize, as it requires an atomic hydrogen source and subtraction of background molecular hydrogen contributions [20][21][22]. In contrast, ATI of Xe cannot currently be simulated with ab initio calculations, but is easily measured and has been used extensively to determine the relative CEP, e.g., using the carrier-envelope phase meter (CEPM) [11].…”

Isotope Effect in Tunneling Ionization of Neutral Hydrogen Molecules

“…Quantitatively accurate ab initio theoretical modelling of this process so far has only been demonstrated for atomic hydrogen [2]. In all other cases the modelling involves a number of approximations, i.e.…”