Name: direct imaging of laser-driven ultrafast molecular rotation
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This work was supported in part by grants-in-aid KAKENHI from the Japan Society for the Promotion of Science (JSPS) and the Ministry of Education, Culture, Sports, Science, and Technology (MEXT) Japan (#26104539, #26620020, #26810011, #15H03766, #15KT0060, #16H00826, and #16K13927); the Konica Minolta Science and Technology Foundation; the "Planting Seeds for Research" program of TokyoTech; the Imaging Science Project of the Center for Novel Science Initiatives (CNSI) at the National Institutes of Natural Sciences (NINS) (#IS261006); the RIKEN-IMS joint program on "Extreme Photonics;" and the Consortium for Photon Science and Technology (CPhoST).

NOTE: Through this protocol, we clarify what we actually did to develop the present method. Exact parameters, including chamber and optical setup design and the sizes and types of the parts, are not always essential to apply the present system to the reader&#39;s apparatus. The essence of the procedures will be given as notes in each step.
1. Construction of a 2D-slice Imaging Apparatus
NOTE: Throughout this step, all the commercially available parts and equipment, su...

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The present procedure enables us to capture a real-time movie of molecular rotation with a slit-based 2D imaging setup. Because the observed ions pass through the slit, step 1.5 is one of the critical steps. The edges of the slit blades must be sharp. When there is a small defect, such as a 0.3 mm dent in the slit, a scratch is observed in the ion image (Figure 6). In such a case, the slit blade should be polished with 2,000-grit wet sandpaper.
Apart from the unique camera ang...

For a deeper understanding and better use of the dynamic nature of molecules, it is essential to clearly visualize molecular motions of interest. Time-resolved Coulomb explosion imaging is one of the powerful approaches to achieve this objective1,2,3. In this approach, the molecular dynamics of interest are initiated by a pump ultrashort laser field and are then probed by a time-delayed probe pulse. Upon probe irradiation, molecules are multiply ionized and broken into fragment ions due to the Coulomb repulsion. The spatial distribution of the ejected ions is a measure of the molecular structure and spatial orientation at the probe irradiation. A sequence of measurement scanning the pump-probe delay time leads to the creation of a molecular movie. It is noteworthy that, for the simplest case &#8211; diatomic molecules &#8211; the angular distribution of the ejected ions directly reflects the molecular axis distribution (i.e., the squared rotational wave function).
With regard to the pump process, recent progress in the coherent control of molecular motion using ultrashort laser fields has led to the creation of highly controlled rotational wave packets4,5. Furthermore, the direction of rotation can be actively controlled by using a polarization-controlled laser field6,7,8. It has therefore been expected that a detailed picture of molecular rotation, including wave natures, could be visualized when the Coulomb explosion imaging technique is combined with such a pump process9,10,11,12,13. However, we sometimes encounter experimental difficulties associated with the existing imaging methods, as mentioned below. The purpose of this paper is to present a new way of overcoming these difficulties and of creating a high-quality movie of molecular rotational wave packets. The first experimental movie of molecular rotation taken with the present method, along with its physical implications, were presented in our previous paper11. The background of development, the detailed theoretical aspect of the present imaging technique, and a comparison with other existing techniques will be given in a forthcoming paper. Here, we will mainly focus on the practical and technical aspects of the procedure, including the combination of the typical pump-probe optical setup and the new imaging apparatus. As in the previous paper, the target system is unidirectionally rotating nitrogen molecules11.
The main experimental difficulty of the existing imaging setup, schematically shown in Figure 1, has to do with the position of the detector, or the camera angle. Because the rotational axis coincides with the laser propagation axis6,7,8 in laser-field-induced molecular rotation, it is not practical to install a detector along the rotational axis. When the detector is installed so as to avoid laser irradiation, the camera angle corresponds to a side observation of rotation. In this case, it is impossible to reconstruct the original orientation of molecules from the projected (2D) ion image14. A 3D imaging detector14,15,16,17,18,19, with which the arrival time to the top detector and the ion impact positions can be measured, offered a unique way to directly observe molecular rotation using Coulomb explosion imaging10,12. However, the acceptable ion counts per laser shot are low (typically &#60; 10 ions) in the 3D detector, meaning that it is difficult to create a long movie of molecular motion with high image quality14. The dead time of the detectors (typically ns) also affects the image resolution and imaging efficiency. It is also not a simple task to make a good pump-probe beam overlap by monitoring a real-time ion image with a laser repetition rate of &#60;~1 kHz. Although several groups have observed rotational wave packets using the 3D technique, the spatial information was limited and/or direct, and a detailed visualization of wave nature, including complicated nodal structures, was not achieved10,12.
The essence of the new imaging technique is the use of the &#34;new camera angle&#34; in Figure 1. In this configuration, laser beam exposure to a detector is avoided while the 2D detector is parallel to the rotational plane, leading to the observation from the rotational axis direction. The slit allows only the ion in the rotational plane (the polarization plane of the laser pulses) to contribute to an image. A 2D detector, which offers a higher count rate (typically ~100 ions) than a 3D detector, can be used. The setup of the electronics is simpler than in the case of 3D detection, while the measurement efficiency is higher. Time-consuming mathematical reconstruction, such as Abel inversion14, is also not needed to extract angular information. These features lead to the easy optimization of the measurement system and to the production of high-quality movies. A standard 2D/3D charged-particle imaging apparatus can be easily modified to the present setup without the use of expensive equipment.

<table border="0" cellpadding="0" cellspacing="0" width="618">
	<tbody>
		<tr height="42">
			<td height="42" style="height:42px;width:249px;">CMOS camera</td>
			<td style="width:83px;">Toshiba TELI</td>
			<td style="width:119px;">BU-238M-ES</td>
			<td style="width:167px;">equipped with SONY IMX174 sensor</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:249px;">High voltage switch</td>
			<td style="width:83px;">Behlke</td>
			<td style="width:119px;">HTS-41-03-GSM</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:249px;">High voltage switch</td>
			<td style="width:83px;">Behlke</td>
			<td style="width:119px;">HTS-80-03</td>
			<td></td>
		</tr>
		<tr height="63">
			<td height="63" style="height:63px;width:249px;">Digital delay generator</td>
			<td style="width:83px;">Stanford research systems</td>
			<td style="width:119px;">DG535</td>
			<td></td>
		</tr>
		<tr height="63">
			<td height="63" style="height:63px;width:249px;">Digital delay generator</td>
			<td style="width:83px;">Stanford research systems</td>
			<td style="width:119px;">DG645</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:249px;">Microchannel plate</td>
			<td style="width:83px;">Photonis</td>
			<td align="right" style="width:119px;">3075</td>
			<td></td>
		</tr>
		<tr height="42">
			<td height="42" style="height:42px;width:249px;">Pulsed valve</td>
			<td style="width:83px;">LAMID LTD</td>
			<td style="width:119px;">Even-Lavie valve&#160;</td>
			<td style="width:167px;">High repetition, room temperature model</td>
		</tr>
		<tr height="84">
			<td height="84" style="height:84px;width:249px;">Molecular beam skimmers</td>
			<td style="width:83px;">Institute for Molecular Science</td>
			<td style="width:119px;">13C11</td>
			<td style="width:167px;">3 and 1.5 mm center hole, 25 degrees full inner angle, and ~50 mm length</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:249px;">Optical Comparator</td>
			<td style="width:83px;">Nikon</td>
			<td style="width:119px;">V-24B</td>
			<td style="width:167px;"></td>
		</tr>
		<tr height="42">
			<td height="42" style="height:42px;width:249px;">DPSS laser</td>
			<td style="width:83px;">Lighthouse Photonics</td>
			<td style="width:119px;">Sprout</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:249px;">Femtosecond Ti:Sapphire oscillator</td>
			<td style="width:83px;">KMLabs</td>
			<td style="width:119px;">Halcyon</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:249px;">Femtosecond Ti:Sapphire amplifier</td>
			<td style="width:83px;">Quantronix</td>
			<td style="width:119px;">Odin-II HE</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:249px;">Motorized linear stage</td>
			<td style="width:83px;">Sigma Koki</td>
			<td style="width:119px;">KST(GS)-100X</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:249px;">Manual X-stage</td>
			<td style="width:83px;">Sigma Koki</td>
			<td style="width:119px;">TSD-601S</td>
			<td></td>
		</tr>
		<tr height="42">
			<td height="42" style="height:42px;width:249px;">High resolution mirror mount</td>
			<td style="width:83px;">Newport</td>
			<td style="width:119px;">Suprema SX100-F2KN-254</td>
			<td></td>
		</tr>
		<tr height="42">
			<td height="42" style="height:42px;width:249px;">High resolution mirror mount</td>
			<td style="width:83px;">LIOP-TEC GmbH</td>
			<td style="width:119px;">SR100-100R-2-HS</td>
			<td></td>
		</tr>
		<tr height="63">
			<td height="63" style="height:63px;width:249px;">Polarization checker</td>
			<td style="width:83px;">Paradigm Devices, Inc.</td>
			<td style="width:119px;">O-tool VIS</td>
			<td></td>
		</tr>
		<tr height="44">
			<td height="44" style="height:44px;width:249px;">Instrument communication interface</td>
			<td style="width:83px;">National Instruments</td>
			<td style="width:119px;">NI-MAX</td>
			<td></td>
		</tr>
		<tr height="45">
			<td height="45" style="height:45px;width:249px;">Graphical development environment for measurement programs</td>
			<td style="width:83px;">National Instruments</td>
			<td style="width:119px;">LabVIEW 2014</td>
			<td></td>
		</tr>
		<tr height="42">
			<td height="42" style="height:42px;width:249px;">Laser line dielectric mirror</td>
			<td style="width:83px;">CVI/LEO</td>
			<td style="width:119px;">TLM2-400/800-45UNP</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:249px;">Laser line dielectric mirror</td>
			<td style="width:83px;">Altechna</td>
			<td colspan="2">Low GDD Ultrafast mirror</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:249px;">Laser line dielectric mirror</td>
			<td style="width:83px;">Altechna</td>
			<td colspan="2">Low GDD Ultrafast mirror</td>
		</tr>
		<tr height="42">
			<td height="42" style="height:42px;width:249px;">Femtosecond polarizer</td>
			<td style="width:83px;">Advanced Thin Films</td>
			<td style="width:119px;">PBS-GVD</td>
			<td></td>
		</tr>
	</tbody>
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direct imaging of laser-driven ultrafast molecular rotation

We present a method for visualizing laser-induced, ultrafast molecular rotational wave packet dynamics. We have developed a new 2-dimensional Coulomb explosion imaging setup in which a hitherto-impractical camera angle is realized. In our imaging technique, diatomic molecules are irradiated with a circularly polarized strong laser pulse. The ejected atomic ions are accelerated perpendicularly to the laser propagation. The ions lying in the laser polarization plane are selected through the use of a mechanical slit and imaged with a high-throughput, 2-dimensional detector installed parallel to the polarization plane. Because a circularly polarized (isotropic) Coulomb exploding pulse is used, the observed angular distribution of the ejected ions directly corresponds to the squared rotational wave function at the time of the pulse irradiation. To create a real-time movie of molecular rotation, the present imaging technique is combined with a femtosecond pump-probe optical setup in which the pump pulses create unidirectionally rotating molecular ensembles. Due to the high image throughput of our detection system, the pump-probe experimental condition can be easily optimized by monitoring a real-time snapshot. As a result, the quality of the observed movie is sufficiently high for visualizing the detailed wave nature of motion. We also note that the present technique can be implemented in existing standard ion imaging setups, offering a new camera angle or viewpoint for the molecular systems without the need for extensive modification.

Figure 4A shows a probe-only raw image of the N2+ ion ejected upon probe irradiation (Coulomb explosion), taken for one probe laser shot. Each bright spot corresponds to one ion. Figure 4B shows a summed image of 10,000 binarized raw camera images. These images show that our imaging setup can monitor the molecules of all the orientation angles in the polarization plane. Figure 4C shows the normalized polar plot correspo...

Watch this Scientific Journal Video about Direct Imaging of Laser-driven Ultrafast Molecular Rotation at JoVE.com

Direct Imaging of Laser-driven Ultrafast Molecular Rotation

We present a protocol for creating a real-time movie of a molecular rotational wave packet using a high-resolution Coulomb explosion imaging setup.

We present a method for visualizing laser-induced, ultrafast molecular rotational wave packet dynamics. We have developed a new 2-dimensional Coulomb ...

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.

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