2.1. STRUCTURAL INFORMATION
The resulting evolution for typical experimental conditions, for the nitrogen molecule, is shown in Figure 2.1, generated with the code in [105]. For each molecule, at the start of the process a state {J, M} is populated. Interaction with the ultrashort aligning pulse populates many adjacent states. The ro-tational wavepacket propagates freely after the laser pulse is turned off. The phase evolution of many rotational states leads to their periodic phasing and de-phasing, referred to as rotational revivals. The results presented in Figure 2.1 are averaged over the incoherent ensemble of initial states populated ac-cording to the Boltzmann distribution. During this evolution hcos2θi(t) will have regular maxima and minima, which mean that the distribution is aligned along laser polarization (maxima) or in the plane orthogonal to it (minima).
The width of the peaks depends on the rotational temperature of the ensemble and the degree of coherence induced between the excited states by the aligning pulse.
Importantly, typical laser pulses with duration of about 100 fsec and peak intensity of some 1013W/cm2 are very efficient in aligning molecules. The typical depth of the aligning potential well can easily reach 50 meV without ionizing or exciting the molecule vibrationally. This is a very strong aligning potential, given that the thermal energy at the room temperature is only about 25 meV. Thus, high degree of molecular alignment is possible, especially using molecular cooling as they are expanded into the vacuum through a nozzle of a jet, under pressure [99].
2.1.3 High harmonic generation in aligned molecules:
insufficiency of the SAE approximation with a single hydrogenic orbital for the description of the experiment in complex molecules such as alkanes.
The first quantum mechanical description of the ionization dependence on the full range of angles was presented in [110]. There, ionization of the molecular hydrogen in a strong low-frequency field was studied, specifically, the depen-dence of the ionization rates on the internuclear distance for states with dif-ferent symmetry, and also the dependence on the angle between the molecular axis and the laser polarization vector. As a consequence of these dependen-cies, the high harmonic generation process should also depend on the molecular alignment angle and on the properties of the ionizing orbital, from which the electron leaves the molecule.
The first observation of HHG in laser-aligned molecular ensemble was made in [103], and further in [111]. For the first time, laser-induced alignment was achieved at relatively high densities of the molecular gas, ∼1017 cm−3, which is a key prerequisite for HHG. The molecules studied experimentally and nu-merically were CS2 and N2. Both experimental and simulated results showed strong dependence of the signal on the molecular alignment angle. Increase in intensity of the HHG signal when the generating pulse was preceded by the alignment pulse, relative to the signal without the alignment pulse was shown for CS2 molecules. Suppression of the signal for the N2 molecules aligned perpendicularly to the HHG generating pulse was also found.
Multiple further studies showed, that HHG spectrum in aligned molecules is strongly affected by the structure of the molecular orbitals, involved in the process. The alignment dependence of HHG in the carbon dioxide molecule was studied for the first time in [112]. The maximum yield of harmonics was found to be strongly dependent on the harmonic number at intermediate angles between 0◦ and 90◦, with a minimum in the yield atθ = 0◦. The overall minimum at θ = 0◦ is consistent with the symmetry of the highest occupied molecular orbital of CO2. The latter has a nodal plane in theθ= 0◦ direction.
The presence of the nodal plane suppresses both ionization and recombination amplitudes.
The influence of the symmetry of the molecular orbital on the HHG was then shown in [113, 114] via theoretical comparison of HHG in aligned N2 and O2
2.1. STRUCTURAL INFORMATION
molecules, that are characterized by different symmetry of the highest occupied molecular orbital: σg for the N2 andπg for the O2. Experimental confirmation of different harmonic modulation depending on the alignment angle, for N2 and O2, is presented in [115].
As the next step in the investigation of the molecular orbital dependence of HHG, more complex molecules were examined. Polyatomic molecules such as allene and acetylene were studied in [116]. The expansion of this study is presented in[117], where additionally HHG in aligned ethylene was observed.
Experimental results were found to be in a good agreement with theoretical calculations based on the strong field approximation (SFA), showing strong dependence of the harmonic yield on the molecular alignment angle. With changing the alignment angle HHG spectrum is modulated for all harmonic numbers, but the modulation is not the same for different harmonics. This is a characteristic feature of HHG in aligned molecules. It is a manifestation of the dependence of the recombination dipole on the energy of the returning electron.
Ionization potentials of some of the small molecules like CO2, N2 or O2 are relatively high (∼13 eV), or, in other words, the number of obtained harmonics for the values of the pump laser parameters, typically used in experiments, is sufficient to study HHG across a broad range of the harmonic photon energies.
However, most larger molecules, such as those investigated in [116] and [117], have smaller ionization potentials Ip ∼ 9−10 eV. This significantly limits the driving laser intensity that can be applied to the molecule before it is fully ionized, keeping the intensity well below 1014 W/cm2 and dramatically reducing the position of the HHG cutoff Ωmax'Ip+ 3.2Up.
This problem can be resolved by increasing the wavelength λ of the driving laser, since Up ∝ λ2. The constraint is the wavelength-dependence of the HHG efficiency. The electron return probability is reduced with growing λ due to the large transverse spread of the re-colliding electronic wavepacket.
The wavepacket spreading between ionization and recombination scales lin-early with the laser period for each of the three spatial dimensions, leading to λ−3 drop in the signal due to the spreading alone. Combined with λ2 growth of the harmonic spectral width and∝λ growth in the density of the harmonic
lines, this leads to the overallλ−6 decrease in the harmonic signal, for a single-molecule response [118]. This signal loss can be partially offset by increasing the density of the molecular gas and by optimizing the phase-matching condi-tions, see e.g. [119] and references therein for discussion.
These conditions were accounted for in the experiment [120], where optimal laser wave length was found to be around 1300 nm (and up to 1800 nm) for molecules N2, CO2, C2H2 and N2O. An extended study of CO2 at the longer wavelengths and different intensities of the driving laser was done later in the work [121], which is considered later in chapter.