Direct Imaging of Laser-driven Ultrafast Molecular Rotation

Overall, the goal of this experiment is to take high resolution, real time snapshots of ultrafast molecular rotation. This method can help answer key questions in the field of Quantum Mechanics and Physical Chemistry such as the connection between the quantum molecular motion and the classical mechanics world. The main advantage of this technique is that we can carry out high resolution, high throughput realtime imaging of molecules from a camera angle which has not been previously realized.

This method can provide insight into rotation dynamics of molecules. It can also be applied to other systems such as oriented molecules and photo resource actions. The equipment needed for the experiment has two main components.

The first is a two dimensional slice imaging apparatus constructed in a vacuum chamber. This schematic provides an idea of what is inside the chamber. After lasers, ionized diatomic molecules.

There are ion optical elements to guide the ionization products. There is a slit to select ions in the laser polarization plane and an electrode that acts as a pulsed repeller. It directs ions to a parallel detector consisting of microchannel plates backed by a phosphor screen.

The second component is the pump probe optical setup. As depicted in the schematic, the setup employs a titanium sapphire laser amplifier to create three colinear femptosecond pulses. The first pulse is linearly polarized with a center wavelength of 820 nanometers and is for molecular alignment.

The second pulse if for direction control. It is a delayed replica of the first pulse, except the linear polarization is tilted 45 degrees with respect to the first. The third pulse is the Coulomb explosion imaging probe.

It is at 407 nanometers, circularly polarized, and lasts 100 femtoseconds. These three pulses enter the vacuum chamber along the same path. An important step is to adjust the polarization state of the pulses using a polarization checker.

This checker allows visualization of the polarization state by the polarization angle dependent transmission intensity. Install the polarization checker in the beam path just before it enters the chamber. Focus on the polarization state of the imaging probe.

The goal is circular polarization. To change the polarization, use the wave plates in the pulse's path. Adjust the wave plates in their rotational optic mounts.

Stop when the checker indicates the polarization is circular. Follow a similar procedure with pulse one by adjusting its wave plates. In the end, its polarization angle should be vertical.

Manipulate the wave plates of the second pulse to produce linear polarization where the angle is at 45 degrees with respect to that of pulse one. Remove the polarization checker and dumps from the beam path and complete other setup preparations. The time overlap of the pulses must be roughly optimized.

To do this, four elements should be mounted just before the focusing lens in the system. The first in the path is an optical window as thick as the focusing lens and the chamber window combined. Next is a nonlinear crystal with a nonlinear response only when the pump and probe interact simultaneously.

Third along the path is a dispersion prism. Finally, there's a sheet of white paper to register the nonlinear signal. Work with the pump one beam and the probe pulse by placing a beam dump on pump two beam.

Search for a third harmonic signal using the motorized stage. Press the move button on the stage controller to scan and search for the third signal. The paper in the beam path will indicate the harmonic signal with a white blue fluorescence.

Next, unblock pump two and block pump one with a beam dumper. Use the manual stage in the pump two line to adjust the beam path. Scan the micrometer based stage until the third harmonic emission occurs.

Determination of the time zero or temporal overlap is complete when fluorescence is visible on the paper. Before continuing, remove all of the elements that were added to the beam line to find the temporal overlap. The unique aspect of the measurement setup is the camera placement.

As suggested in this schematic, the detector surface is perpendicular to the laser polarization plane. The camera axis is perpendicular to the detector's surface and thus parallel to the line of the laser propagation. Install a digital camera on the optical post in front of the vacuum view port ensuring it is properly oriented.

In addition, install a cooling fan behind the camera. To reduce unwanted light, drape a curtain over the region between the view port and the lens. With the camera connected to the computer, start the control software and maximize the gain of the camera.

Start capturing images resulting from the molecular ionization. The camera images should cover the entire surface of the detector when the camera is properly positioned. Continue to monitor the captured images to adjust the camera focus.

The size of the spots due to the ions should be as sharp and as small as possible. For measurements, first find the signal and optimize the ion imaging settings. To do this, use only the probe pulse by blocking the pump pulse beams.

The pulse travels through a plano convex lens to focus it on the molecular beams. A pulse generator sets the time of the high voltage switches for the ion optics. Set it to the estimated arrival time of the Nitrogen molecule ions.

Monitor the ion image while adjusting the lens position and gas pulse time to achieve the largest and brightest image. Next use the pulse generator to change the time of the high voltage switches. Use the time for the Coulomb exploded doubly ionized Nitrogen channel.

At the computer, decrease the camera frame rate and increase the exposure time. In addition, adjust the ion optic biases so the observed ion distribution becomes an undistorted ellipse. Move on to finding the pump probe spatial overlap.

Unblock pump one, but keep pump two blocked. Make use of the telescope to position the pump pulse beam waste in high resolution mirror mount one to alter the beam position. As the telescope is adjusted, monitor the spot due to the pump pulses.

When the smallest spot size is seen, the beam waste is in the molecular beam. Continue by ensuring the probe pulse images the molecules irradiated by the pump pulse. This requires moving the delay stage from its zero position.

For Nitrogen, move the motorized stage about 600 micrometers forward. As in this example, the previously isotropic image should become strongly anisotropic along the direction of pump one polarization. At this point, block pump one and unblock pump two.

Use high resolution mirror mount two to find the pump probe overlap. Adjust the mirror while keeping the optical path of pump one unchanged. The alignment should be along the oblique angle of pump two's polarization direction with respect to the polarization of pump one.

To observe the unidirectional rotation dynamics, unblock both pump beams. Use the manual delay stage to set the delay between the pump beams. Adjust the stage to four picoseconds in the case of molecular Nitrogen.

Set up the motorized stage in the probe beam to scan probe delays. Check to determine if unidirectional rotation can be recognized in camera images as the probe delay is scanned. For a movie of unidirectional molecular rotation change the camera frame rate and exposure time.

Use computer control to collect data and step through probe time values. This is a probe only raw image of doubly ionized Nitrogen that was ejected after one shot of probe irradiation. Each bright spot corresponds to an ion.

This image is produced when 10, 000 such binarized images are summed over. The real space size is 80 millimeters by 50 millimeters. Note that false color is used to show signal intensity.

Ultimately the data can be used to generate a normalized polar plot with the radial value proportional to an angle dependent probability. Clear evidence of unidirectional molecular rotation is presented in this series of selected snapshots taken after irradiation with two pump pulses. There are three series.

One has the ion image in which the elliptical shape has been converted to a circle. The second has a corresponding polar plot. Finally, there is a series created with a model constructed using overlapped images of dumbbells at various orientation angle, weighted with observed angular probabilities.

After it's debutment, this technique paved the ways for researcher in the field of molecular science to explore the quantum nature of molecular rotation. After watching this video, you should have a good understanding of how to control and visualize the ultrafast molecular rotation using phaser gun laser pulses.