physics Physics is the natural science that studies matter, its fundamental constituents, its motion and behavior through space and time, and the related entities of energy and force. "Physical science is that department of knowledge which ...

and

physical chemistry Physical chemistry is the study of macroscopic and microscopic phenomena in chemical systems in terms of the principles, practices, and concepts of physics such as motion, energy, force, time, thermodynamics, quantum chemistry, statistica ...

, time-resolved spectroscopy is the study of dynamic processes in

material Material is a substance or mixture of substances that constitutes an object. Materials can be pure or impure, living or non-living matter. Materials can be classified on the basis of their physical and chemical properties, or on their geolo ...

s or

chemical compound A chemical compound is a chemical substance composed of many identical molecules (or molecular entities) containing atoms from more than one chemical element held together by chemical bonds. A molecule consisting of atoms of only one element ...

s 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

laser A laser is a device that emits light through a process of optical amplification based on the stimulated emission of electromagnetic radiation. The word "laser" is an acronym for "light amplification by stimulated emission of radiation". The ...

s, 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.

Transient-absorption spectroscopy

Transient-absorption spectroscopy (TAS), also known as flash photolysis, is an extension of

absorption spectroscopy Absorption spectroscopy refers to spectroscopic techniques that measure the absorption of radiation, as a function of frequency or wavelength, due to its interaction with a sample. The sample absorbs energy, i.e., photons, from the radiating fi ...

. Ultrafast transient absorption spectroscopy, an example of non-linear spectroscopy, measures changes in the

absorbance Absorbance is defined as "the logarithm of the ratio of incident to transmitted radiant power through a sample (excluding the effects on cell walls)". Alternatively, for samples which scatter light, absorbance may be defined as "the negative lo ...

transmittance Transmittance of the surface of a material is its effectiveness in transmitting radiant energy. It is the fraction of incident electromagnetic power that is transmitted through a sample, in contrast to the transmission coefficient, which is t ...

in the sample. Here, the absorbance at a particular

wavelength In physics, the wavelength is the spatial period of a periodic wave—the distance over which the wave's shape repeats. It is the distance between consecutive corresponding points of the same phase on the wave, such as two adjacent crests, tr ...

or range of wavelengths of a sample is measured as a function of time after

excitation Excitation, excite, exciting, or excitement may refer to: * Excitation (magnetic), provided with an electrical generator or alternator * Excite Ballpark, located in San Jose, California * Excite (web portal), web portal owned by IAC * Electron ...

by a flash of light. In a typical experiment, both the light for excitation ('pump') and the light for measuring the absorbance ('probe') are generated by a pulsed laser. If the process under study is slow, then the time resolution can be obtained with a continuous (i.e., not pulsed) probe beam and repeated conventional spectrophotometric techniques. Time-resolved absorption spectroscopy relies on our ability to resolve two physical actions in real time. The shorter the detection time, the better the resolution. This leads to the idea that femtosecond laser based spectroscopy offers better resolution than nano-second laser based spectroscopy. In a typical experimental set up, a pump pulse excites the sample and later, a delayed probe pulse strikes the sample. In order to maintain the maximum spectral distribution, two pulses are derived from the same source. The impact of the probe pulse on the sample is recorded and analyzed with wavelength/ time to study the dynamics of the excited state. Absorbance (after pump)-Absorbance (before pump)= Δ Absorbance Δ Absorbance records any change in the absorption spectrum as a function of time and wavelength. As a matter of fact, it reflects ground state bleaching (-ΔA), further excitation of the excited electrons to higher excited states(+ΔA), stimulated emission(-ΔA) or product absorption(+ΔA). Bleaching of ground state refers to depletion of the ground state carriers to excited states. Stimulated emission follows the fluorescence spectrum of the molecule and is Stokes shifted relative to and often still overlaps with the bleach signal. This is a lasing effect (coherent emission) of the excited dye molecules under the strong probe light. This emission signal cannot be distinguished from the absorption signal and often gives false negative Δ absorbance peaks in the final spectra that can be decoupled via approximations. 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 Photoluminescence (abbreviated as PL) is light emission from any form of matter after the absorption of photons (electromagnetic radiation). It is one of many forms of luminescence (light emission) and is initiated by photoexcitation (i.e. photo ...

. Transient absorption can be measured as a function of

time Time is the continued sequence of existence and event (philosophy), events that occurs in an apparently irreversible process, irreversible succession from the past, through the present, into the future. It is a component quantity of various me ...

. 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.

Conditions

TA measurements are highly sensitive to laser repetition rate, pulse duration, emission wavelength, polarization, intensity, sample

chemistry Chemistry is the scientific study of the properties and behavior of matter. It is a natural science that covers the elements that make up matter to the compounds made of atoms, molecules and ions: their composition, structure, proper ...

, solvents,

concentration In chemistry, concentration is the abundance of a constituent divided by the total volume of a mixture. Several types of mathematical description can be distinguished: '' mass concentration'', '' molar concentration'', ''number concentration'', ...

and

temperature Temperature is a physical quantity that expresses quantitatively the perceptions of hotness and coldness. Temperature is measured with a thermometer. Thermometers are calibrated in various temperature scales that historically have relied o ...

. 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.

Application

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.

Other multiple-pulse techniques

Transient spectroscopy as discussed above is a technique that involves two pulses. There are many more techniques that employ two or more pulses, such as: * Photon echoes. *

Four-wave mixing Four-wave mixing (FWM) is an intermodulation phenomenon in nonlinear optics, whereby interactions between two or three wavelengths produce two or one new wavelengths. It is similar to the third-order intercept point in electrical systems. Four-wave ...

(involves three laser pulses) * fifth-order experiments (involves four excitation pulses and a probe pulse) The interpretation of experimental data from these techniques is usually much more complicated than in transient-absorption spectroscopy.

Nuclear magnetic resonance Nuclear magnetic resonance (NMR) is a physical phenomenon in which nuclei in a strong constant magnetic field are perturbed by a weak oscillating magnetic field (in the near field) and respond by producing an electromagnetic signal with a ...

and

electron spin resonance Electron paramagnetic resonance (EPR) or electron spin resonance (ESR) spectroscopy is a method for studying materials that have unpaired electrons. The basic concepts of EPR are analogous to those of nuclear magnetic resonance (NMR), but the sp ...

are often implemented with multiple-pulse techniques, though with radio waves and micro waves instead of visible light.

Time-resolved infrared spectroscopy

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 fluorescence spectroscopy

Time-resolved fluorescence spectroscopy is an extension of

fluorescence spectroscopy Fluorescence spectroscopy (also known as fluorimetry or spectrofluorometry) is a type of electromagnetic spectroscopy that analyzes fluorescence from a sample. It involves using a beam of light, usually ultraviolet light, that excites the electro ...

. Here, the

fluorescence Fluorescence is the emission of light by a substance that has absorbed light or other electromagnetic radiation. It is a form of luminescence. In most cases, the emitted light has a longer wavelength, and therefore a lower photon energy, tha ...

of a sample is monitored as a function of time after excitation by a flash of light. The time resolution can be obtained in a number of ways, depending on the required sensitivity and time resolution: * With fast-detection electronics (nanoseconds and slower) * With Time Correlated Single Photon Counting, TCSPC (picoseconds and slower) * With a

streak camera A streak camera is an instrument for measuring the variation in a pulse of light's intensity with time. They are used to measure the pulse duration of some ultrafast laser systems and for applications such as time-resolved spectroscopy and LID ...

(picoseconds and slower) * With intensified CCD (ICCD) cameras (down to 200 picoseconds and slower) * With optical gating (femtoseconds-nanoseconds) - a short laser pulse acts as a

gate A gate or gateway is a point of entry to or from a space enclosed by walls. The word derived from old Norse "gat" meaning road or path; But other terms include ''yett and port''. The concept originally referred to the gap or hole in the wall ...

for the detection of fluorescence light; only fluorescence light that arrives at the detector at the same time as the gate pulse is detected. This technique has the best time resolution, but the efficiency is rather low. An extension of this optical gating technique is to use a "Kerr gate", which allows the scattered Raman signal to be collected before the (slower) fluorescence signal overwhelms it. This technique can greatly improve the signal:noise ratio of Raman spectra. This technique uses convolution integral to calculate a lifetime from a fluorescence decay.

Time-resolved photoemission spectroscopy and 2PPE

Time-resolved photoemission spectroscopyA. Stolow, A. E. Bragg, and D. M. Neumark, Femtosecond time-resolved photoelectron spectroscopy, Chem Rev, 104 (2004) 171

/ref> and Two-photon photoelectron spectroscopy, two-photon photoelectron spectroscopy (2PPE) are important extensions to

photoemission spectroscopy Photoemission spectroscopy (PES), also known as photoelectron spectroscopy, refers to energy measurement of electrons emitted from solids, gases or liquids by the photoelectric effect, in order to determine the binding energies of electrons in t ...

. These methods employ a pump-probe setup. In most cases the pump and probe are both generated by a pulsed

and in the UV region. The pump excites the atom or molecule of interest, and the probe ionizes it. The

electrons The electron ( or ) is a subatomic particle with a negative one elementary electric charge. Electrons belong to the first generation of the lepton particle family, and are generally thought to be elementary particles because they have n ...

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.

References

{{Branches of Spectroscopy