Measurement of Ultrafast Vibrational Coherences in Polyatomic Radical Cations with Strong-Field Adiabatic Ionization

Summary

We present a protocol for probing ultrafast vibrational coherences in polyatomic radical cations that result in molecular dissociation.

Abstract

We present a pump-probe method for preparing vibrational coherences in polyatomic radical cations and probing their ultrafast dynamics. By shifting the wavelength of the strong-field ionizing pump pulse from the commonly used 800 nm into the near-infrared (1200-1600 nm), the contribution of adiabatic electron tunneling to the ionization process increases relative to multiphoton absorption. Adiabatic ionization results in predominant population of the ground electronic state of the ion upon electron removal, which effectively prepares a coherent vibrational state ("wave packet") amenable to subsequent excitation. In our experiments, the coherent vibrational dynamics are probed with a weak-field 800 nm pulse and the time-dependent yields of dissociation products measured in a time-of-flight mass spectrometer. We present the measurements on the molecule dimethyl methylphosphonate (DMMP) to illustrate how using 1500 nm pulses for excitation enhances the amplitude of coherent oscillations in ion yields by a factor of 10 as compared to 800 nm pulses. This protocol may be implemented in existing pump-probe setups through the incorporation of an optical parametric amplifier (OPA) for wavelength conversion.

Introduction

Since the invention of the laser in 1960's, the goal of selectively breaking chemical bonds in molecules has been a longstanding dream of chemists and physicists. The ability to tune both laser frequency and intensity was believed to enable direct cleavage of a target bond through selective energy absorbance at the associated vibrational frequency^1,2,3,4. However, early experiments found that intramolecular vibrational redistribution of the absorbed energy throughout the molecule often resulted in non-selective cleavage of the weakest bo....

Protocol

NOTE: All commercially acquired instruments and parts such as the laser, vacuum pumps, chamber, time-of-flight tube and microchannel plate detector were installed and operated according to the manufacturer’s instructions or user’s manual. Laser safety goggles designed for the operating laser intensities and wavelengths should be worn.

1. Construction of TOF-MS²⁶

1. Design and build an ultrahigh vacuum (UHV) chamber that has enough space to accommodate a standard stack of ion optics²⁶ and provisions to mount optical windows on 2 ¾” flanges on either side of the i....

Results

Results obtained for the molecule DMMP²¹ are presented. Figure 3 shows the DMMP mass spectra taken at zero time-delay with the peak intensities of the 1500 nm pump and 800 nm probe pulses being 8 x 10¹³ and 8 x 10¹² W cm^-2, respectively. For reference, the mass spectrum taken with only the pump pulse is also shown. The spectra are averaged over 10,000 laser shots (total acquisition time 12 s). The incre.......

Discussion

This protocol enables us to resolve ultrafast vibrational dynamics in polyatomic radical cations through selective preparation of the ions in the ground electronic state. While the standard strong-field ionization procedure using 800 nm can prepare vibrational coherences in ground-electronic state radical cations of first-row diatomics^10,11,12,13 and CO₂¹⁴

Materials

List of materials used in this article

Name	Company	Catalog Number	Comments
Mass spectrometer components
TOF lens stack and flight tube assembly	Jordan TOF Products, Inc.	C-677
18 mm Z-gap detector assembly	Jordan TOF Products, Inc.	C-701Z
TOF high voltage power supply	Jordan TOF Products, Inc.	D-603
Vacuum system components
Rotary vane backing pump	Edwards Vacuum LLC	RV12
Turbomolecular pumps (2)	Edwards Vacuum LLC	EXT255H
Turbomolecular pump controllers (2)	Edwards Vacuum LLC	EXC300
Pressure gauge	Edwards Vacuum LLC	AIGX-S-DN40CF
Chiller for water cooling	Neslab	CFT-25
Femtosecond laser system
Ti:Sapphire regenerative amplifier	Coherent, Inc.	Astrella	oscillator and amplifier in a single integrated system
Optical Parametric Amplifer (OPA)	Light Conversion	TOPAS Prime
Motion control
Motorized linear translation stage 1" travel	Thorlabs	Z825B
controller for linear translation stage	Thorlabs	KDC 101
USB controller hub and power supply	Thorlabs	KCH 601
Manual linear translation stage 1" travel	Thorlabs	PT1
Detectors
Pyroelectric laser energy meter	Coherent, Inc.	1168337
Thermal laser power meter	Coherent, Inc.	5356E16R
Si-biased detector 200-1100 nm	Thorlabs	DET10A
Compact USB CMOS Camera	Thorlabs	DCC1545M
USB spectrometer	Ocean Optics	HR4000
1 GHz digital oscilloscope	LeCroy	WaveRunner 610Zi
Optics
Type 1 BBO crystal	Crylight Photonics	BBO007	aperture and thickness may be customized
Achromatic half wave plate, 1100-2000 nm	Thorlabs	AHWP05M-1600
Wollaston prism polarizer	Thorlabs	WPM10
Hollow retro-reflector	PLX, Inc.	OW-20-1C
Variable neutral density filter	Thorlabs	NDC-100C-2
Longpass dichroic mirror 2" diameter	Thorlabs	DMLP950L
Software
Digital Camera image software	Thorlabs	ThorCam
Instrument communication interface	National Instruments	NI-MAX
Graphical development environment for measurement programs	National Instruments	LabVIEW
Data processing software	Mathworks	MATLAB