Direct Imaging of Laser-driven Ultrafast Molecular Rotation

Summary

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

Abstract

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.

Introduction

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 objective^1,2,3. 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....

Protocol

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'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, such as a vacuum pump and a detector, are installed according to the manufacturer's instructions or user's manuals.

1. As in the construction of ....

Results

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.......

Discussion

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.......

Materials

List of materials used in this article

Name	Company	Catalog Number	Comments
CMOS camera	Toshiba TELI	BU-238M-ES	equipped with SONY IMX174 sensor
High voltage switch	Behlke	HTS-41-03-GSM
High voltage switch	Behlke	HTS-80-03
Digital delay generator	Stanford research systems	DG535
Digital delay generator	Stanford research systems	DG645
Microchannel plate	Photonis	3075
Pulsed valve	LAMID LTD	Even-Lavie valve	High repetition, room temperature model
Molecular beam skimmers	Institute for Molecular Science	13C11	3 and 1.5 mm center hole, 25 degrees full inner angle, and ~50 mm length
Optical Comparator	Nikon	V-24B
DPSS laser	Lighthouse Photonics	Sprout
Femtosecond Ti:Sapphire oscillator	KMLabs	Halcyon
Femtosecond Ti:Sapphire amplifier	Quantronix	Odin-II HE
Motorized linear stage	Sigma Koki	KST(GS)-100X
Manual X-stage	Sigma Koki	TSD-601S
High resolution mirror mount	Newport	Suprema SX100-F2KN-254
High resolution mirror mount	LIOP-TEC GmbH	SR100-100R-2-HS
Polarization checker	Paradigm Devices, Inc.	O-tool VIS
Instrument communication interface	National Instruments	NI-MAX
Graphical development environment for measurement programs	National Instruments	LabVIEW 2014
Laser line dielectric mirror	CVI/LEO	TLM2-400/800-45UNP
Laser line dielectric mirror	Altechna	Low GDD Ultrafast mirror
Laser line dielectric mirror	Altechna	Low GDD Ultrafast mirror
Femtosecond polarizer	Advanced Thin Films	PBS-GVD