Ultra fast laser spectroscopy
Encyclopedia
Ultra-fast laser spectroscopy is the study of molecules on extremely short time scales (nanoseconds to femtoseconds) after their excitation with a pulsed laser
. This method is used extensively to examine the energy states and electron dynamics of any molecule whose reaction to light is of interest. Many different procedures have been developed; some common methods are: ultra-fast transient absorption (TA); time-correlated single photon counting (TCSPC); and time-resolved photo-electron spectroscopy (TRPES). All methods must take into consideration the quantum-mechanical nature of absorption and fluorescence, specifically that individual molecules, even in pure samples, will not emit their photons simultaneously even though they are excited simultaneously, and the rate of decay between the two states is related to the difference in energy between them (see Atomic spectral line
s for more information).
This is done by splitting a pulsed laser beam into two paths. A pulse along one path travels to a photomultiplier
tube (PMT), while another path travels through the sample. The first pulse is detected by a photomultiplier tube, which activates a time-to-amplitude converter (TAC) circuit. This circuit begins to build a charge on a capacitor
which will only be discharged once the PMT sends another electrical pulse to the circuit. This electrical pulse comes after the second laser pulse excites the molecule to a higher energy state, and a photon is eventually emitted from a single molecule upon returning to its original state. Thus, the longer a molecule takes to emit a photon, the higher the voltage of the resulting pulse. The central concept of this method is that only a single photon is needed to discharge the capacitor. Thus, this experiment must be repeated many times to gather the full range of delays between excitation and emission of a photon. After each trial, a pre-calibrated computer converts the voltage sent out by the TAC into a time and records the event in a histogram
of time since excitation. Since the probability that no molecule will have relaxed decreases with time, a decay curve emerges that can then be analyzed to find out the decay rate of the event.
A major complicating factor is that many decay processes involve multiple energy states, and thus multiple rate constants. Though non-linear least squared analysis can usually detect the different rate constants, determining the processes involved is often very difficult and requires the combination of multiple ultra-fast techniques. Even more complicating is the presence of inter-system crossing and other non-radiative processes in a molecule. A limiting factor of this technique is that it is limited to studying energy states that result in fluorescent decay.
is used to excite a molecule's electrons from their ground state
s to higher-energy excited state
s. A probing light source, typically a xenon arc lamp
, is used to obtain an absorption spectrum of the compound at various times following its excitation. As the excited molecules absorb the second pulse, they are further excited to even higher states. After passing through the sample, the light from the arc lamp continues to an avalanche photodiode
array, and the data is processed to generate an absorption spectrum of the excited state. Since all the molecules in the sample will not undergo the same dynamics simultaneously, this experiment must be carried out many times, and the data must be averaged in order to generate spectrums with accurate intensities and peaks. Unlike TCSPC, this technique can be carried out on non-fluorescent samples.
pulse ionizes the molecule. The kinetic energy
of the electron
s from this process are then detected, through various methods including energy mapping, time of flight measurements etc. As above, the process is repeated many times, with different time delays between the probe pulse and the pump pulse. This builds up a picture of how the molecule relaxes over time.
A variation of this method looks at the positive ions created in this process, and is called time-resolved photo-ion spectroscopy (TRPIS)
Laser
A laser is a device that emits light through a process of optical amplification based on the stimulated emission of photons. The term "laser" originated as an acronym for Light Amplification by Stimulated Emission of Radiation...
. This method is used extensively to examine the energy states and electron dynamics of any molecule whose reaction to light is of interest. Many different procedures have been developed; some common methods are: ultra-fast transient absorption (TA); time-correlated single photon counting (TCSPC); and time-resolved photo-electron spectroscopy (TRPES). All methods must take into consideration the quantum-mechanical nature of absorption and fluorescence, specifically that individual molecules, even in pure samples, will not emit their photons simultaneously even though they are excited simultaneously, and the rate of decay between the two states is related to the difference in energy between them (see Atomic spectral line
Atomic spectral line
In physics, atomic spectral lines are of two types:* An emission line is formed when an electron makes a transition from a particular discrete energy level of an atom, to a lower energy state, emitting a photon of a particular energy and wavelength...
s for more information).
Time-correlated single photon counting
This method is used to analyze the relaxation of molecules from an excited state to a lower energy state. Since various molecules in a sample will emit photons at different times following their simultaneous excitation, the decay must be thought of as having a certain rate rather than occurring at a specific time after excitation. By observing how long individual molecules take to emit their photons, and then combining all these data points, an intensity vs. time graph can be generated that displays the exponential decay curve typical to these processes. However, it is difficult to simultaneously monitor multiple molecules. Instead, individual excitation-relaxation events are recorded and then averaged to generate the curve.This is done by splitting a pulsed laser beam into two paths. A pulse along one path travels to a photomultiplier
Photomultiplier
Photomultiplier tubes , members of the class of vacuum tubes, and more specifically phototubes, are extremely sensitive detectors of light in the ultraviolet, visible, and near-infrared ranges of the electromagnetic spectrum...
tube (PMT), while another path travels through the sample. The first pulse is detected by a photomultiplier tube, which activates a time-to-amplitude converter (TAC) circuit. This circuit begins to build a charge on a capacitor
Capacitor
A capacitor is a passive two-terminal electrical component used to store energy in an electric field. The forms of practical capacitors vary widely, but all contain at least two electrical conductors separated by a dielectric ; for example, one common construction consists of metal foils separated...
which will only be discharged once the PMT sends another electrical pulse to the circuit. This electrical pulse comes after the second laser pulse excites the molecule to a higher energy state, and a photon is eventually emitted from a single molecule upon returning to its original state. Thus, the longer a molecule takes to emit a photon, the higher the voltage of the resulting pulse. The central concept of this method is that only a single photon is needed to discharge the capacitor. Thus, this experiment must be repeated many times to gather the full range of delays between excitation and emission of a photon. After each trial, a pre-calibrated computer converts the voltage sent out by the TAC into a time and records the event in a histogram
Histogram
In statistics, a histogram is a graphical representation showing a visual impression of the distribution of data. It is an estimate of the probability distribution of a continuous variable and was first introduced by Karl Pearson...
of time since excitation. Since the probability that no molecule will have relaxed decreases with time, a decay curve emerges that can then be analyzed to find out the decay rate of the event.
A major complicating factor is that many decay processes involve multiple energy states, and thus multiple rate constants. Though non-linear least squared analysis can usually detect the different rate constants, determining the processes involved is often very difficult and requires the combination of multiple ultra-fast techniques. Even more complicating is the presence of inter-system crossing and other non-radiative processes in a molecule. A limiting factor of this technique is that it is limited to studying energy states that result in fluorescent decay.
Ultra-fast transient absorption
This method is typical of 'pulse-probe' experiments, where a pulsed laserLaser
A laser is a device that emits light through a process of optical amplification based on the stimulated emission of photons. The term "laser" originated as an acronym for Light Amplification by Stimulated Emission of Radiation...
is used to excite a molecule's electrons from their ground state
Ground state
The ground state of a quantum mechanical system is its lowest-energy state; the energy of the ground state is known as the zero-point energy of the system. An excited state is any state with energy greater than the ground state...
s to higher-energy excited state
Excited state
Excitation is an elevation in energy level above an arbitrary baseline energy state. In physics there is a specific technical definition for energy level which is often associated with an atom being excited to an excited state....
s. A probing light source, typically a xenon arc lamp
Xenon arc lamp
A xenon arc lamp is a specialized type of gas discharge lamp, an electric light that produces light by passing electricity through ionized xenon gas at high pressure to produce a bright white light that closely mimics natural sunlight...
, is used to obtain an absorption spectrum of the compound at various times following its excitation. As the excited molecules absorb the second pulse, they are further excited to even higher states. After passing through the sample, the light from the arc lamp continues to an avalanche photodiode
Avalanche photodiode
An avalanche photodiode is a highly sensitive semiconductor electronic device that exploits the photoelectric effect to convert light to electricity. APDs can be thought of as photodetectors that provide a built-in first stage of gain through avalanche multiplication. From a functional standpoint,...
array, and the data is processed to generate an absorption spectrum of the excited state. Since all the molecules in the sample will not undergo the same dynamics simultaneously, this experiment must be carried out many times, and the data must be averaged in order to generate spectrums with accurate intensities and peaks. Unlike TCSPC, this technique can be carried out on non-fluorescent samples.
Time-resolved photo-electron spectroscopy
This method is very similar to Ultra-fast transient absorption, the difference being that the second laserLaser
A laser is a device that emits light through a process of optical amplification based on the stimulated emission of photons. The term "laser" originated as an acronym for Light Amplification by Stimulated Emission of Radiation...
pulse ionizes the molecule. The kinetic energy
Kinetic energy
The kinetic energy of an object is the energy which it possesses due to its motion.It is defined as the work needed to accelerate a body of a given mass from rest to its stated velocity. Having gained this energy during its acceleration, the body maintains this kinetic energy unless its speed changes...
of the electron
Electron
The electron is a subatomic particle with a negative elementary electric charge. It has no known components or substructure; in other words, it is generally thought to be an elementary particle. An electron has a mass that is approximately 1/1836 that of the proton...
s from this process are then detected, through various methods including energy mapping, time of flight measurements etc. As above, the process is repeated many times, with different time delays between the probe pulse and the pump pulse. This builds up a picture of how the molecule relaxes over time.
A variation of this method looks at the positive ions created in this process, and is called time-resolved photo-ion spectroscopy (TRPIS)
See also
- SpectroscopySpectroscopySpectroscopy is the study of the interaction between matter and radiated energy. Historically, spectroscopy originated through the study of visible light dispersed according to its wavelength, e.g., by a prism. Later the concept was expanded greatly to comprise any interaction with radiative...
- Time-resolved spectroscopyTime-resolved spectroscopyIn 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 that occur after illumination of a material, but in principle, the technique can be applied to...
- Terahertz time-domain spectroscopy (THz-TDS)
- Electronic configuration
- Atomic spectral lineAtomic spectral lineIn physics, atomic spectral lines are of two types:* An emission line is formed when an electron makes a transition from a particular discrete energy level of an atom, to a lower energy state, emitting a photon of a particular energy and wavelength...
- FluorescenceFluorescenceFluorescence is the emission of light by a substance that has absorbed light or other electromagnetic radiation of a different wavelength. It is a form of luminescence. In most cases, emitted light has a longer wavelength, and therefore lower energy, than the absorbed radiation...
External links
- Why use TCSPC?
- Ultrafast studies of single semiconductor and metal nanostructures through transient absorption microscopy, a Chemical Science mini review by Gregory Hartland