Skip to main content
SolarWiki

6: Pump Probe Spectroscopy

Introduction

Ultrafast laser spectroscopy has been a boon to the study of chemical and atomic fast processes (<10-10s) in the fields of  biophysics, physics, material science, and chemistry[]. The most basic and widely used technique in ultrafast laser spectroscopy is known as pump-probe spectroscopy[]. Pump-probe spectroscopy is popular because there are few detectors that have a temporal resolution in the femtosecond range[]. Using pump-probe spectroscopy, however, the temporal resolution becomes dependent not on the measurement speed of the detector but on the temporal size of the pump and probe pulses. Commercially available lasers exist with twenty femtosecond output pulses[] and using nonlinear techniques, pulses that are eight attoseconds(10^-18s) long can be produced in the XUV wavelength range[].

In pump-probe spectroscopy one laser pulse is used as a pump which moves the sample into an excited state while another laser pulse is used to measure the spectrum and/or the kinetics of the excited state. There are a number of varied configurations in which this type of experiment is carried out and the pump and probe pulses may be modified as necessary to suit the experiment.  The data generated from ultrafast spectroscopy measurements allows the determination of the types and lifetimes of the excited states in the material[].

Experimental Procedure

For a pump-probe experiment there are four main components: light source(s), pump pulse, probe pulse, and detection system. The light source for an ultrafast pump-probe experiment is almost always an ultrafast laser. This is because the temporal resolution of the experiment is limited by the temporal size of the pump and probe pulses[] and that ultrafast pulses are ideal for nonlinear processes due to their high intensity, which allows for the efficient manipulation of the pulse frequency through doubling[], tripling[]continuum generation[], or down-conversion[]. These processes allow for great variation in frequency and frequency range of the pump and probe pulses which lend great versatility to the technique of pump-probe spectroscopy.​

 

Pumpprobediagram.jpg

 

Above is a diagram of a rather basic broadband pump-probe set-up, the emitted ultrafast light is split into the probe and pump beams which are then recombined at the sample, in both space and time, before that though the probe beam is broadened through the use of a supercontinuum crystal. The moveable micrometer stage causes the probe pulse to arrive later and later relative to the pump pulse which allows the measurement ultrafast kinetics. The chopper in the pump line blocks every other pulse so that half the probe pulses that go through hit the unexcited sample and can be used as a reference.  

Data

In the simplest pump-probe spectroscopy experiments what is being measured is the increase or decrease of the probe pulse energy as a function of the frequency of the probe pulse and the time after the pump pulse has excited the sample. The data from these experiments are generally measured in two ways, either in the change in optical density ({Delta}OD) or in differential transmission ({Delta}T/T). The results are generally presented either as spectra at specific times(Figure 1(a-c)) or kinetics at specific wavelengths(Figure 1(d-f)). Figure 1c shows both positive (~500 nm to 600 nm) and negative (~375 nm to 500 nm) bands. The negative band corresponds to a bleach of the ground state by the pump pulse[], though negative bands can also correspond to stimulated emission by the sample[]. The positive band corresponds to excited state absorption by one or more of the excited species in the sample[].

Figure 2. A) The transient spectra of CdZnS nanoparticles. B) The transient spectra of CdZnS nanoparticles with ZnS shells. C) The Transient spectra of CdZnS/ZnS nanoparticles with Pd adducts. D-F) Kinetics of the three different nanoparticles at three different probe wavelengths(412nm, 450nm, 600nm).

 

Advanced Techniques

Due to its versatility they are numerous advanced spectroscopic techniques that can be considered variations on the pump-probe method. These advanced techniques include polarization spectroscopy[], in which either the probe or pump is composed of polarized photons allowing the measurement of sample anisotropy in either the ground or excited state. There is also multidimensional pump-probe[] spectroscopy which uses multiple pump beams to excite the sample. Then there are various types of ultrafast Raman[] spectroscopy, which relies on the inelastic scattering of the probe beam leading to up or down shifted emitted photons. Also we have ultrafast pump-probe spectroscopy that involve wavelengths far longer than the visible such as terahertz spectroscopy[], microwave spectroscopy[], and far infrared spectroscopy[], these techniques are often used to measure the rotation and vibration spectra of molecules. Finally, ultrafast photon-electron spectroscopy[] uses a powerful probe pulse which ionizes the excited electrons which are then measured.