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Experimental and analytical details
This protocol is extracted from research article:
Deformation of an inner valence molecular orbital in ethanol by an intense laser field
Sci Adv, May 17, 2019;

Procedure

A linearly polarized Ti:Sapphire laser pulse (~60 fs, ~800 nm, and ~140 μJ) was converted into a circularly polarized pulse by passing through an achromatic quarter wave plate and was focused on an effusive ethanol beam with an off-axis parabolic mirror (f = 200 mm). The effusive beam of deuterated ethanol CH3CD2OH vapor was continuously supplied into a vacuum chamber through a microsyringe (70-μm inner diameter) and a skimmer (0.2-mm orifice diameter; Beam Dynamics model 2). The base pressure of the chamber without the sample was below 1 × 10−8 Pa.

The three-dimensional momentum vectors of an electron and an ion from an identical molecule were measured in coincidence (18, 21). Ions and electrons created in the focal region were accelerated by an electrostatic lens (28) toward two microchannel plate detectors with delay line position encoding (RoentDek HEX80) on opposite ends of the vacuum chamber. Two-dimensional positions and time of flights were recorded using time-to-digital converters with a resolution of 25 ps (RoentDek TDC8HP). The laser repetition rate was 1 kHz, and the detection count rate was set to be less than 0.3 counts per laser shot.

The measured ion and electron momentum vectors ($p→ion$ and $p→ele$) were used to obtain the relative angle φrel between the projected vectors onto the polarization plane ($p→ionpol$ and $p→elepol$ shown in fig. S1C). Laboratory frame electron momentum distributions have a torus shape, indicating that electrons are emitted mainly along the polarization plane (fig. S1, A and B). This electron motion suggests that the ionization proceeds in the tunnel ionization regime (25). RFPAD of the tunnel ionization was derived with φrel by taking account of the electron drift by the circularly polarized laser field ($p→elepol$$p→tunnel$). Defining φrel to be positive in going in the same direction as the E-field rotation, we obtained the RFPAD as a function of φRFPAD = φrel − 90° (fig. S1C). The laser intensity was determined by a least-squares fit to a theoretical expression of the electron momentum distribution in a circularly polarized laser field (29).

The laboratory frame momentum distributions of the CD2OH+ and CH3CD2+ fragment ions are almost isotropic and have a peak at the center (pion = 0) (fig. S2, A and B). The orientation of the parent molecule was determined from the direction of the fragment recoil based on the axial recoil approximation. In the present analysis, we selectively analyzed the coincidence events for the fragment ions satisfying the following two conditions. The first condition was that the out-of-plane angle of the ion emission with respect to the polarization plane was smaller than 10° ($aionout‐of‐plane$ < 10°) (see also fig. S2D). This condition extracted the events producing the fragment ion recoiling along the polarization plane. The other condition concerned the fragment ion velocity υion, which is the sum of the recoil velocity and the initial velocity of the parent molecule in the thermal distribution. The initial velocity spread of the parent molecule blurred fragment recoil direction from which the orientation of the parent molecule was determined. The velocity distribution of the parent molecule was approximated by that of the parent ion (fig. S2, A and B), because the recoil momentum [<1 atomic unit (au) as shown in fig. S1A] given by the electron tunneling and the circularly polarized laser field is smaller than the initial momentum spread of the parent ion [$ΔpCH3CD2OH+jet$= 14.3 au and $ΔpCH3CD2OH+TOF$= 2.4 au in full width at half maximum (FWHM)]. The velocity distribution indicated that most of the parent ions were slower than 2 × 10−4 au in the present experiment. Therefore, we imposed the condition of υion ≥ 2 × 10−4 au (fig. S2, C and D).

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