Уақыт бойынша ажыратылатын спектроскопия: Материалдар мен химиялық қосылыстардағы динамикалық процестерді зерттеу

Spectroscopic technique
In physics and physical chemistry, time resolved spectroscopy is the study of dynamic processes in materials or chemical compounds by means of spectroscopic techniques. Most often, processes are studied after the illumination of a material occurs, but in principle, the technique can be applied to any process that leads to a change in properties of a material. With the help of pulsed lasers, it is possible to study processes that occur on time scales as short as 10−16 seconds. All time resolved spectra are suitable to be analyzed using the two dimensional correlation method for a correlation map between the peaks. Product absorption refers to any absorption changes caused due to formation of intermediate reaction products. TA measurements can also be used to predict non emissive states and dark states unlike time resolved photoluminescence. Transient absorption can be measured as a function of wavelength or time. The TA curve along wavelength provides information regarding evolution/decay of various intermediate species involved in chemical reaction at different wavelengths. The transient absorption decay curve against time contains information regarding the number of decay processes involved at a given wavelength, how fast or slow the decay processes are. It can provide evidences with respect to inter system crossing, intermediate unstable electronic states, trap states, surface states etc.

TA measurements are highly sensitive to laser repetition rate, pulse duration, emission wavelength, polarization, intensity, sample chemistry, solvents, concentration and temperature. The excitation density (no. of photons per unit area per second) must be kept low; otherwise, sample annihilation, saturation and orientational saturation may come into play.

Transient absorption spectroscopy helps study the mechanistic and kinetic details of chemical processes occurring on the time scales of few picoseconds to femto seconds. These chemical events are initiated by an ultrafast laser pulse and are further probed by a probe pulse. With the help of TA measurements, one can look into non radiative relaxation of higher electronic states (~femtoseconds), vibrational relaxations (~picoseconds) and radiative relaxation of excited singlet state (occurs typically on nanoseconds time scale). Transient absorption spectroscopy can be used to trace the intermediate states in a photo chemical reaction; energy, charge or electron transfer process; conformational changes, thermal relaxation, fluorescence or phosphorescence processes, optical gain spectroscopy of semiconductor laser materials. etc. With the availability of UV Vis NIR ultrafast lasers, one can selectively excite a portion of any large molecule to desired excited states to study the specific molecular dynamics. Transient absorption spectroscopy has become an important tool for characterizing various electronic states and energy transfer processes in nanoparticles, to locate trap states and further helps in characterizing the efficient passivation strategies.

Time resolved infrared (TRIR) spectroscopy also employs a two pulse, "pump probe" methodology. The pump pulse is typically in the UV region and is often generated by a high powered Nd:YAG laser, whereas the probe beam is in the infrared region. This technique currently operates down to the picosecond time regime and surpasses transient absorption and emission spectroscopy by providing structural information on the excited state kinetics of both dark and emissive states.

Time resolved photoemission spectroscopy and two photon photoelectron spectroscopy (2PPE) are important extensions to photoemission spectroscopy. These methods employ a pump probe setup. In most cases the pump and probe are both generated by a pulsed laser and in the UV region. The pump excites the atom or molecule of interest, and the probe ionizes it. The electrons or positive ions resulting from this event are then detected. As the time delay between the pump and the probe are changed, the change in the energy (and sometimes emission direction) of the photo products is observed. In some cases multiple photons of a lower energy are used as the ionizing probe.