Maryam Tarazkar

Ultraviolet surprise: Efficient soft x-ray high-harmonic generation in multiply ionized plasmas.

Short wavelengths birth shorter ones The shortest laser pulses—with durations measured in attoseconds—arise from a process termed high-harmonic generation (HHG). Essentially, a longer, “driving” pulse draws electrons out of gaseous atoms like a slingshot, and, when they ricochet back, light emerges at shorter wavelengths. Most HHG has been carried out using light near the visible/infrared boundary for the driving pulse. Popmintchev et al. used an ultraviolet driving pulse instead, which yielded an unexpectedly efficient outcome. These results could presage a more generally efficient means of creating x-ray pulses for fundamental dynamics studies as well as technological applications. Science, this issue p. 1225 Ultraviolet pulses show unexpected efficiency in generating the higher-frequency emission underlying attosecond spectroscopy. High-harmonic generation is a universal response of matter to strong femtosecond laser fields, coherently upconverting light to much shorter wavelengths. Optimizing the conversion of laser light into soft x-rays typically demands a trade-off between two competing factors. Because of reduced quantum diffusion of the radiating electron wave function, the emission from each species is highest when a short-wavelength ultraviolet driving laser is used. However, phase matching—the constructive addition of x-ray waves from a large number of atoms—favors longer-wavelength mid-infrared lasers. We identified a regime of high-harmonic generation driven by 40-cycle ultraviolet lasers in waveguides that can generate bright beams in the soft x-ray region of the spectrum, up to photon energies of 280 electron volts. Surprisingly, the high ultraviolet refractive indices of both neutral atoms and ions enabled effective phase matching, even in a multiply ionized plasma. We observed harmonics with very narrow linewidths, while calculations show that the x-rays emerge as nearly time-bandwidth–limited pulse trains of ~100 attoseconds.

Measurement of an Electronic Resonance in a Ground-State, Gas-Phase Acetophenone Cation via Strong-Field Mass Spectrometry

A one-photon ionic resonance is measured in the strong-field regime in acetophenone by recording the mass spectra as a function of excitation wavelength from 800 to 1500 nm. The ratio of the benzoyl to parent ion signals in the mass spectrum varies significantly with excitation wavelength, where the highest ratio observed upon excitation at 1370 nm (0.90 eV) indicates the presence of a one-photon resonance. At the resonant wavelength, the ratio of the benzoyl to parent ion signals increases linearly with laser intensity over a range from 1.1 × 10(13) to 6.0 × 10(13) W cm(-2). The ratio increases by a factor of 5 at 1370 nm with increasing pulse duration from 60 to 100 fs. Calculations using the equation of motion coupled cluster method support the existence of a one-photon transition from the ground ionic to a dissociative excited ionic state (0.87 eV), where motion of the acetyl group from a planar to nonplanar structure within the pulse duration enables the otherwise forbidden transition.

Measurement of ionic resonances in alkyl phenyl ketone cations via infrared strong field mass spectrometry.

Strong-field excitation of alkyl phenyl ketone molecules reveals an electronic resonance at 1370 nm in the radical cations upon measuring mass spectra as a function of excitation wavelength from 1240 to 1550 nm. The ratio of the benzoyl fragment ion to parent ion signal in acetophenone increases from 1:1.5 at 1240 nm excitation to 5:1 at 1370 nm (0.9 eV), and back to 1:1 at 1450 nm. Unlike acetophenone and propiophenone, the homologous molecules acetone and ethylbenzene exhibit no wavelength-dependent fragmentation patterns over the range from 1240 to 1550 nm, supporting the hypothesis that the electronic structure of the alkyl phenyl ketone cation enables the one-photon transition. Calculations on the acetophenone and propiophenone radical cations show the existence of a bright state, D2, 0.87 and 0.88 eV, respectively, above the ground-state D0 minimum. Calculations of the potential energy surfaces of the acetophenone radical cation suggest that a D2 → D0 radiationless transition precedes dissociation on D0. Upon population transfer to the D2 surface, the wavepacket motion is directed toward a three-state conical intersection (D0/D1/D2) that facilitates the photodissociation by converting electronic to vibrational energy on the D0 surface.

Strong Field Adiabatic Ionization Prepares a Launch State for Coherent Control.

We demonstrate that excitation of acetophenone with a strong field, near-infrared femtosecond pulse (1150-1500 nm) results in adiabatic ionization, producing acetophenone radical cation in the ground electronic state. The time-resolved transients of the parent and fragment ions probed with a weak 790 nm pulse reveal an order of magnitude enhancement of the peak-to-peak amplitude oscillations, ∼ 100 fs longer coherence time, and an order of magnitude increase in the ratio of parent to fragment ions in comparison with nonadiabatic ionization with a strong field 790 nm pulse. Equation of motion coupled cluster and classical wavepacket trajectory calculations support the mechanism wherein the probe pulse excites a wavepacket on the ground surface D0 to the excited D2 surface at a delay of 325 fs, resulting in dissociation to the benzoyl ion. Direct population transfer to the D2 state within the duration of a 1370 nm pump pulse eliminates wavepacket oscillation on the D0 state.

Controlling the dissociation dynamics of acetophenone radical cation through excitation of ground and excited state wavepackets

Time-resolved measurements of the acetophenone radical cation prepared via adiabatic ionization with strong field 1270 nm excitation reveal coupled wavepacket dynamics that depend on the intensity of the 790 nm probe pulse. At probe intensities below W cm−2, out of phase oscillations between the parent molecular ion and the benzoyl fragment ion are shown to arise from a one-photon excitation from the ground D0 ionic surface to the D1 and/or D2 excited surfaces by the probe pulse. At higher probe intensities, a second set of wavepacket dynamics are observed that couple the benzoyl ion to the phenyl, butadienyl, and acylium fragment ions. Equation of motion coupled cluster calculations of the ten lowest lying ionic surfaces and the dipole couplings between the ground ionic surface D0 and the nine excited states enable elucidation of the dissociation pathways and deduction of potential dissociation mechanisms. The results can lead to improved control schemes for selective dissociation of the acetophenone radical cation.

Higher-order nonlinearity of refractive index: The case of argon

The nonlinear coefficients, n4, of the time-dependent refractive index for argon are calculated in the non-resonant optical regime. Second-order polynomial fitting of DC-Kerr, γ((2))(-ω; ω, 0, 0), electric field induced second harmonic generation (ESHG), γ((2))(-2ω; ω, ω, 0), and static second-order hyperpolarizability, γ((2))(0; 0, 0, 0), is performed using an auxiliary electric field approach to obtain the corresponding fourth-order optical properties. A number of basis sets are investigated for the fourth-order hyperpolarizability processes at 800 nm at coupled cluster singles and doubles level of theory, starting with the t-aug-cc-pV5Z basis set and expanding that basis set by adding diffuse functions and polarization functions. Comparison shows that the results obtained with the t-aug-cc-pV5Z basis are in very good agreement with the results obtained using the q-aug-cc-pV5Z, t-aug-cc-pV6Z, and q-aug-cc-pV6Z basis sets. To calculate the nonlinear refractive index n4, an approximate formula is suggested which expresses the related degenerate six-wave mixing coefficient, γ((4))(-ω; ω, -ω, ω, -ω, ω), in terms of the DC-Kerr, γ((4))(-ω; ω, 0, 0, 0, 0), ESHG, γ((4))(-2ω; ω, ω, 0, 0, 0), and the static fourth-order hyperpolarizability coefficients. The higher-order nonlinear refractive index n4 is found to be positive over the wavelengths 300 nm-2000 nm. In the infrared spectral range, the obtained values of n4 are in qualitative agreement with the results of Kramers-Kronig-based calculations.

Theoretical study of second-order hyperpolarizability for nitrogen radical cation

We report calculations of the static and dynamic hyperpolarizabilities of the nitrogen radical cation in doublet state. The electronic contributions were computed analytically using density functional theory and multi-configurational self-consistent field method with extended basis sets for non-resonant excitation. The open-shell electronic system of nitrogen radical cation provides negative second-order optical nonlinearity, suggesting that the hyperpolarizability coefficient, in the non-resonant regime is mainly composed of combinations of virtual one-photon transitions rather than two-photon transitions. The second-order optical properties of nitrogen radical cation have been calculated as a function of bond length starting with the neutral molecular geometry (S0 minimum) and stretching the N–N triple bond, reaching the ionic D0 relaxed geometry all the way toward dissociation limit, to investigate the effect of internuclear bond distance on second-order hyperpolarizability.