Picosecond Transient Absorption Spectroscopy System

PicoTAS– Picosecond to Second Transient Absorption Spectrometer

About Unisoku/TII Group

As a member of Tokyo Instruments, Inc. (TII Group), UNISOKU has been developing, manufacturing and custom designing the world’s cutting-edge research instruments since 1974. It was chosen as one of Japan’s “Global Niche Top 100 Companies” by the Japanese Ministry of Economy, Trade and Industry in March 2014. It is striving to be the leading company in Japan as “No.1 in Nano-Technology measurement and Photonics”.

PicoTAS Transient Absorption Spectroscopy Systems


PicoTAS is one of UNISOKU’s newest innovative product capable of measuring Picosecond to Second Transient Absorption Spectrometer. It uses the combination of Asynchronous Pump Light and Probe Light technique and the Randomly Interleaved Pulse Train (RIPT) method. This newly patented technology allows seamless bridge of the picosecond to nanosecond time range, eliminates the notorious 1 ns – 20 ns time gap, avoids the pitfalls and limitations of optical delays in traditional pump and probe methods and nanosecond flash photolysis.

Unisoku has been developing this product in cooperation with Nihon University, Osaka University and Meijo University in the framework of the Japan Science and Technology Agency’s “Development of Systems and Technologies for Advanced Measurement and Analysis (JST-SENTAN)” program.

Outstanding Features

  1. Unisoku’s patented RIPT technique eliminates deficiencies of conventional TAS systems.
  2.  Asynchronous Operation
  3. Complete coverage of gap time region (1 ns–20 ns)
  4. Broad wavelength range from VIS to NIR
  5. Transient absorption measurement from 100 picosecond to second
  6. Completely eliminates photoluminescence contamination, which has plagued conventional TAS methods.
  7. Can be easily upgraded to include TCSPC lifetime capability
  8.  Compact size: can be placed on lab bench, optical bench is not required (ns model),
  9. Convenient to operate: No complex optical system for alignment, easy to operate, user-friendly software.

What is TAS ( Transient Absorption Spectroscopy)?

Time-resolved spectroscopy is the study of dynamic processes in materials or chemical compounds by means of spectroscopic techniques. Both TAS and Fluorescence lifetime measurement are time resolved methods, but the latter is limited to a fluorescent process at the first stage of the reaction (excited singlet state). TAS, on the other hand, can explore and analyze multi-step complex processes in a wide time region (short-lived radical species, charge transfer states, etc.), to predict emissive, non emissive states and dark states.

Conventional TAS

Conventional TAS usually refers to the pump-probe spectroscopic technique for probing and characterizing the electronic and structural properties of short-lived excited states (transient states) of photochemically or photophysically relevant molecules. In a typical conventional experimental set up, the sample is first excited by a pump pulse (pump light), then struck by a delayed probe pulse (probe light), and the impact of the probe pulse on the sample is observed by measurement of time-resolved absorption change of the probe light, analyzed against wavelength or 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. TAS curve along wavelength provides information of various intermediate species involved in chemical reaction at different wavelengths, and TAS against time, measured at a given wavelength, can provide information such as inter-system crossing, intermediate unstable electronic states, trap states, surface states etc.

PicoTAS from Unisoku

PicoTAS uses the Asynchronous Pump Light and Probe Light technique combined with the Randomly Interleaved Pulse Train (RIPT) method [1], which was featured by Science [2] and Nature Photonics [3]. This is Unisoku’s newly patented technology [4], allows for seamless bridge of the picosecond and nanosecond time ranges, eliminating the notorious 1 ns – 20 ns time gap in conventional TAS instruments.

[1] Nakagawa, T.; Okamoto, K.; Hanada. H.; Katoh, R. Opt. Lett.2016, 41, 1498-1501.
[2] Yeston, J. Science 352, 670 (2016)
[3] Donati, G. Nature Photonics 10, 285 (2016)
[4] US Patent 9,709,497, 2017

What is Randomly Interleaved Pulse Train (RIPT) Method

The RIPT technique involves detecting the waveforms of uncorrelated pump pulses and probe pulse and calculating the time delay between the two. In the RIPT method, signal waveforms of both pump light and probe light are recorded by high-speed detectors on each pump light irradiation. The delay time of a probe pulse after the pump pulse is calculated from these waveforms. Each light intensity of probe light pulse that is transmitted through a sample is recorded by using a detector with amplifier, and the intensity is plotted against the delay time. When pump light irradiation is repeated, the delay time differs every time because pump light and probe light are asynchronous. In this way, the plot generates a continuous curve after many repeats of pump light irradiation. By executing delta optical density calculation, transient absorption curve is reconstructed. This elegant direct method avoids the pitfalls and limitations of optical delays in traditional pump and probe methods and nanosecond flash photolysis.

The main features of RIPT

  • In RIPT method, the ratio between pump /probe light pulses is 1:Many, while this ratio is 1:1 for Pump-Probe method and 1: ∞ for CW method
  • In RIPTI, Pump and Probe are asynchronous
  • In RIPT, Time difference is calculated passively
  • By repetition of pumping cycles with randomly-interleaved-pulse-train probing, TA signal with high time resolution and wide time range is reconstructed by high-speed data processingOptics Letters, 41, 1498 (2016) ; Highlighted in Nat. Photon., 10 (2016), 285; Science, 352 (2016), 669 

What are the Differences between picoTAS and Conventional Techniques?

Eliminate “Gap time Region” (1-20ns):

There are mainly two conventional transient absorption (TA) techniques, Pump & Probe method and Nanosecond Flash Photolysis method. Both have difficulty in measuring the “Gap Time Region” (1‐20 nanoseconds) in which many important phenomena exist.

picoTAS uses Randomly Interleaved Pulse Train (RIPT method). It can measure wide time range of time including the time region from 1 to 20 nanoseconds.

Eliminating the influence of fluorescence:

picoTAS has the capability to eliminate the influence of fluorescence, therefore, not only non-fluorescent but also fluorescent intermediates can be detected and identified correctly. This is accomplished by picking up the signal intensity just before the rise of each probe pulse signal and repeating this procedure for a series of probe pulse train data set, and creating a background curve, which includes a photoluminescence decay signal generated by the pump pulse. Subtracting the background curve from individual probe pulse train data resulting in a transient absorption curve that is free of any photoluminescent contamination 

Models: Two models of PicoTAS are offered, PicoTAS-ns (<400 ps) and PicoTAS-ps (<100 ps) depending on the pulse width of the pump laser. The configurations are flexible and can accommodate user-provided pump laser if it meets the required specs.

Completely Covers Gap Time Region (1 ns ~ 20 ns)

MethodRIPT method (Randomly Interleaved Pulse Train method)
<100 ps
Time Resolution (10%–90% rise time)<400 ps<100 ps
Delay Time Resolution10 ps, 20 ps, 50 ps, 100 ps, 200 ps, 500 ps, 1 ns, 2 ns, 5 ns, 10 ns, 20 ns
Full Scale100 ns–2 ms
Wavelength Range410–1600 nm
Automatic Controlwavelength scan, light intensity adjustment, shutter control
Sample Holdersolution (optical path length is 2 mm), thin film
Pump Light Source
    Sourcepassive Q-SW microchip laserpicosecond mode-locked laser
    Wavelength532 nm and/or 355 nm532 nm and/or 355 nm and/or 266 nm
    Pulse Width<350 ps<25 ps
    Pulse Energy>20 µJ>80 µJ
    Repetition Rate100–1000 Hz (variable)1000 Hz
Probe Light Source
   Sourcepicosecond Supercontinuum Light Source
   Pulse Width<50–100 ps (dependent on wavelength)
   Repetition Rate20 MHz ± 5%
PC & Software
   OSWindows® 7/10
   FunctionAutomatic reconstruction of transient absorption temporal profile, curve fitting (nonlinear least square method), data storage in text format
Installation Environmentno optical bench requiredon optical bench

* Specifications are subject to change without notification

* Our engineers will help to configure your customized system.

Dimensions of Optomechanical Unit

Top View

PicoTAS Top view

Front View

Back View

*Installation spaces for probe light source, PC and oscilloscope are needed separately.
**-ps model further requires an installation space for pump light source. 


1. Wavelength-Tunable Pump Light Source for -ps model

Variable excitation wavelength

Type Optical Parametric Generator (OPG)
Pump Laser Picosecond mode-locked laser 1kHz, 355 nm, 0.3W
Wavelength Range 410-709 nm, 710-2300 nm
Pulse Energy 50mJ@450nm

2. CoolSpeK Low-Temperature Cell Holder (for 2mm cell)

Temperature Range from -180°C to +100°C

Temperature Range -180℃ ~100°C
Temperature Control Flow control of liquid nitrogen by an automatic valve
Functions Low Dew Condensation by Heating of Optical Window, Stirrer
Liquid Nitrogen Reservoir Stainless, 2L
Duration of 2H at -80℃

3. TCSPC Lifetime Capability 

4. Global Analysis Software

Spectrum Analysis of Multicomponent and Various Analysis Functions

Software Globalworks – made by US Company OLIS
Analysis Feature Singular Value Decomposition (SVD),
Global Fitting
OSWindows 10


Customized system with existing pump light source and/or probe light source.

Recommended Specification for Pump Light SourceRepetition rate  1 kHz,
Pulse width <500 ps,
Output energy 20 μJ/pulse
※Please contact us for the source out of the specification above
Recommended Specification for Probe Light Source1. Supercontinuum light source with repetition-rate of 20 MHz
2. Supercontinuum light source equipped with a pulse picker (Settable at 20 MHz.)
3. Picosecond laser diode with high output energy (100nJ/pulse)

Notes: System performance might be limited depending on the specifications of pump light source and/or probe light source.

Please feel free to contact us for the transient absorption system that can eliminate long-lived luminescence by making the most use of the RIPT method and a lower repetition-rate supercontinuum light source

1. picoTAS Optomechanical Unit (Monochrometers, Optics, Sample Chamber, Detectors)

  • 410 – 1600 nm
  • Optical design minimizing chromatic aberration
  • Double Beam Optical System to correct light intensity fluctuation
  • 2 mm Solution Cell Holder with stirrer
  • Thin Film Holder and optional 2-Aeix scanner
  • Oblique Incidence Excitation
  • High excitation efficiency for thin samples
  • Automatic shutter (pump, probe)
  • Probe Light Auto-Balancing Mechanism


2. Pump Light Source

ns model: Pulse Width 350 ps, 1 kHz

ps model: Pulse Width 25 ps, 1 kHz

(Pump Light and Probe Light of picoTAS are Asynchronous)

4. Probe Light Source–High-Repetition-Rate Super Continuum Light Source (50-100 ps, 20 MHz, 410-1600 nm )

4. Dedicated  Oscilloscope

5. PC

The capability to measure wide time range, including the 1–20 nanosecond “Gap Time”  Region”, which was so troublesome for conventional transient absorption methods, combined with its capability to eliminate the influence of fluorescence allows the picoTAS to be used to measure many photo-induced reactions that were not possible to be measured before. The instrument is unrivaled for the following applications:


  • Measuring of Transient Absorption Spectra of Excited Singlet/Triplet State
  • Direct determination of the intersystem crossing (ISC) rate
  • Studies of Electron Transfer, Charge Separation/Recombination Dynamics
    • Electron-hole recombination in TiO2 crystal
  • Observation of Intermolecular Reaction, Excimer formation
  • Characterization of nanomaterials:
    • Identification of reactive species in photoexcited nanocrystalline TiO2 films
    • Laser-induced phase transition in gold nanoparticles
    • Electron dynamics in gold nanocrystals
  • Analysis of Dynamics of Artificial Photosynthesis, Solar Energy Cell, Organic EL:
    • Photosynthetic systems
    • Polymer based thin-film solar cells
    • Charge generation in perovskite solar cells
    • Charge carrier formation in fullerene/polythiophene films
    • Light-harvesting mechanism in polymer/fullerene/dye ternary blends

1. Completely Covers Gap Time Region (1 ns ~ 20 ns)

The conventional methods have difficulty in measuring from 1 to 20 nanoseconds, but picoTAS allows us to measure this “Gap Time Region” which has almost never been studied so far.

In the Gap Time region, there exists various states and processes inside and among molecules.

  • Excited Singlet State:  the initial excited state of many photochemical reaction
  • Excited Triplet State: the second excited state which plays a key role in determining the reaction products
  • Radical Species: highly reactive intermediate state


Benzophenone in Acetonitrile Probe at 550 nm

2. Broadband Wavelength Coverage from VIS to NIR

Recently, near-infrared measurements in transient absorption spectroscopy are highly demanded. As picoTAS uses a super-continuum light source for the probe light source, a broad wavelengths range from visible (> 410 nm) to near-infrared region (< 1600 nm) can be covered seamlessly.

Obtain Transient Absorption Temporal Profile at Every Wavelength

Super-continuum light source used as the probe light source in picoTAS emits light in a broad wavelength range from visible to near-infrared region. Though its light intensity has wavelength dependence, picoTAS can adjust the light intensity automatically which enters into detectors.


Select Time to Extract Transient Absorption Spectra

Display and Save the Spectra


Ir(ppy)3 in THF


3. Measures Wide Time Range from 100 ps to ms

picoTAS  can measure a wide time range. It covers more than 6 orders of magnitude from 100 picosecond to millisecond, from formation to full decay of intermediates.

C60 in Toluene

4. Removes Fluorescence Signal

In conventional nanosecond flash photolysis measurements, luminescence from the sample often interferes with the signal. Since picoTAS has an ingenious way to remove luminescence signal, we can obtain accurate transient absorption (TA) signal of excited singlet and triplet, etc

5. Comparison of Time Resolution(10-90% Rise Time)

picoTAS has 100 times higher time resolution than conventional nanosecond flash photolysis systems. The time resolution (<100 ps for -ps model, <400 ps for -ns model) is comparable to TCSPC fluorescence lifetime measurements, so picoTAS can be widely applied to many reactions including non-luminescent samples.

Observation of the fast (<10 ps ) ISC of palladium porphyrin

Figure Courtesy of Dr. T. Suenobu, Osaka Univ

6. High Sensitive Transient Absorption Measurement of Photocatalyst

To elucidate the photoreaction mechanism of titanium dioxide which shows remarkable behavior as a photocatalytic material, it is known that the pump light intensity must be reduced and detection of very small TA signals is required. In this measurement, picoTAS catches less than 1 mOD changes (equivalent to 0.025% transmittance change) in nanoseconds time region.

Samples Courtesy of Prof. R. Katoh, Nihon Univ.

7. Fullerene (t ISC  ≃ 1 ns)

Fullerene is an promising material for electron acceptors of organic thin-film solar cell and its time constant of intersystem crossing (ISC) is known ~1 ns, so it is difficult to measure TA of this attractive reagent by both pump & probe and nanosecond flash photolysis methods. With the use of picoTAS, the spectra of the excited singlet, ISC and triplet state can be clearly observed from visible to infrared wavelength region. Also, a complete decay curve of the triplet with lifetime of microseconds can be observed.

Pump 355 nm, 25 ps,  500 nm~1250 nm. Minimum Bin 50 ps

Artificial PhotoSynthesis


  • Kamada et al., (2020). Photocatalytic CO2 Reduction Using a Robust Multifunctional Iridium Complex toward the Selective Formation of Formic Acid. J. Am. Chem. Soc. 142, 10261   
  • H. Hong et al., (2019) Photocatalytic Oxygenation Reactions with a Cobalt Porphyrin Complex Using Water as an Oxygen Source and Dioxygen as an Oxidant. J. Am. Chem. Soc. 141, 9155
  • Aratani et al., (2017) Dual function photocatalysis of cyano-bridged heteronuclear metal complexes for water oxidation and two-electron reduction of dioxygen to produce hydrogen peroxide as a solar fuel. Chem. Comm., 53, 3473
  • Tsudaka et al., (2017) Photoinduced Electron Transfer in 9-Substituted 10-Methylacridinium Ions. Chem. – A European J., 23, 1306
  • Aratani et al., (2016) Photocatalytic Hydroxylation of Benzene by Dioxygen to Phenol with a Cyano-Bridged Complex Containing FeII and RuII Incorporated in Mesoporous Silica Alumina. Inorg. Chem., 55, 5780
  • Isaka et al., (2016) Production of hydrogen peroxide by combination of semiconductor-photocatalysed oxidation of water and photocatalytic two-electron reduction of dioxygen. RSC Advances, 6, 42041




  • Shibasaki et al., (2019) Effect of reabsorption of fluorescence on transient absorption measurements. Spectrochimica Acta Part A: Mol and Biom Spect., 220, 117127
  • Yeston, (2016) Mind the (nano) gap. Science, 352, 669
  • Donati, (2016) Temporal flexibility. Nature Photonics, 10, 285
  • Nakagawa et al., (2016) Probing with randomly interleaved pulse train bridges the gap between ultrafast pump-probe and nanosecond flash photolysis. Opt. Lett., 41, 1498


Nano Particles


  • Kawawaki et al., (2019) Carrier-Selective Blocking Layer Synergistically Improves the Plasmonic Enhancement Effect. JACS, 141, 8402
  • Lian et al., (2018) Near infrared light induced plasmonic hot hole transfer at a nano-heterointerface. Nature Comm., 9, 2314
  • Lian et al., (2018) Durian-Shaped CdS@ZnSe Core@Mesoporous-Shell Nanoparticles for Enhanced and Sustainable Photocatalytic Hydrogen Evolution. J. Phys. Chem. Lett., 9, 2212


Single Fission


  • Nakamura et al., (2019) Quantitative Sequential Photoenergy Conversion Process from Singlet Fission to Intermolecular Two-Electron Transfers Utilizing Tetracene Dimer. ACS Energy Lett., 4, 26
  • Sakai et al., (2018) Multiexciton Dynamics Depending on Intramolecular Orientations in Pentacene Dimers: Recombination and Dissociation of Correlated Triplet Pairs. J. Phys. Chem. Lett., 9, 3354
  • Matsui et al., (2019) Exergonic Intramolecular Singlet Fission of an Adamantane-Linked Tetracene Dyad via Twin Quintet Multiexcitons. J. Phys. Chem. C 123, 18813




  • Yu et al., (2018) Excited-State Electronic Properties in Zr-Based Metal-Organic Frameworks as a Function of a Topological Network.J. Am. Chem. Soc., 140, 10488


Nano Ring (Cycloparaphenylenes)


  • Fujitsuka et al., (2019) Size-Dependent Relaxation Processes of Photoexcited [n]Cycloparaphenylenes (n = 5–12): Significant Contribution of Internal Conversion in Smaller Rings. J. Phys. Chem. A, 123, 4737
  • Suenobu et al., (2020) Reaction of Oxygen with the Singlet Excited State of [n]Cycloparaphenylenes (n = 9, 12, and 15): A Time-Resolved Transient Absorption Study Seamlessly Covering Time Ranges from Sub-Nanoseconds to Microseconds by the Randomly-Interleaved-Pulse-Train Method. J. Phys. Chem. A, 124, 46




  • Narushima et al., (2019) Suppressed Triplet Exciton Diffusion Due to Small Orbital Overlap as a Key Design Factor for Ultralong‐Lived Room‐Temperature Phosphorescence in Molecular Crystals. Adv. Mater., 31, 1807268
  • Hirata et al., (2019) Pyrrole‐Based π‐System–PtII Complexes: Chiroptical Properties and Excited‐State Dynamics with Microsecond Triplet Lifetimes. Chem. Eur. J., 25, 8797
  • Hirata, (2018) Intrinsic Analysis of Radiative and Room-Temperature Nonradiative Processes Based on Triplet State Intramolecular Vibrations of Heavy Atom-Free Conjugated Molecules toward Efficient Persistent Room-Temperature Phosphorescence. J. Phys. Chem. Lett., 9, 4251
  • Hirata et al., (2017) Large Reverse Saturable Absorption at the Sunlight Power Level Using the Ultralong Lifetime of Triplet Excitons. J. Phys. Chem. Lett., 8, 3683
  • Zhang et al., (2020) Number of Surface-Attached Acceptors on a Quantum Dot Impacts Energy Transfer and Photon Upconversion Efficiencies. ACS Photonics 7, 1876
  • Tajima et al., (2020) Photoinduced Electron Transfer in a MoS2/Anthracene Mixed-Dimensional Heterojunction in Aqueous Media. B. Chem. Soc. Jpn. 93, 745
  • Hirata et al., (2020) Effect of Tris(Trimethylsilyl)Silyl Group on the Fluorescence and Triplet Yields of Oligothiophenes. J. Phys. Chem. C 124, 3277
  • Ito et al., (2020) Enhancement of Negative Photochromic Properties of Naphthalene‐Bridged Phenoxyl‐Imidazolyl Radical Complex. ChemPhysChem 21, 1578
  • Abe et al., (2020) Structural Transformation of 2‐(p‐Aminophenyl)‐1‐hydroxyinden‐3‐ylmethyl Chromophore as a Photoremovable Protecting Group. ChemPhotoChem DOI: 10.1002/cptc.202000149
  • Nikaido et al., (2019) Delocalization of positive charge in aromatic liquids studied by subnanosecond near-infrared transient absorption spectroscopy. Chem. Phys. Lett. 731, 136578
  • Sakakibara et al., (2020) Intramolecular photoinduced electron transfer reactions of zinc (II) porphyrin dyads studied with a sub-ns time resolution. J. Porphyrins Phthalocyanines DOI:10.1142/S1088424620500224