Transient Gain Spectroscopy Research Articles

Transient gain spectroscopy of (GaIn)As quantum wells: Experiment and microscopic analysis

Transient gain measurements are performed for (GaIn)As quantum well structures. Gain up to gL=2×10−3 per quantum well is observed on a time scale of several hundred picoseconds. A microscopic theory quantitatively provides theoretical support and yields the carrier density as a function of time by matching the spectral shapes and intensities to the experiment. The decay rate of the population density is found to be proportional to the carrier density, which underlines the importance of monomolecular recombination processes.

The band of ND3 studied by laser-induced amplified spontaneous emission and transient gain spectroscopy

Abstract Two nonlinear laser spectroscopic methods have been employed for interrogating the C ′ 1 A ′ 1 → A 1 A 2 ′′ transition of ND 3 . The first method utilized laser-induced amplified spontaneous emission (LIASE) as the signal radiation. When single rovibronic levels in the C ′ 1 A ′ 1 ( 2 n , n=1–7 ) states of the ND 3 molecules were populated through two-photon excitation from the X 1 A 1 state by pulsed UV laser light, yellow LIASE was generated along the input laser axis. The q Q branches with J = K dominated both the C ′ ← X 2 0 0 excitation and C ′ → A 2 1 1 LIASE spectra. LIASE was almost perfectly polarized parallel to the polarization plane of the incident UV radiation. The second method, transient gain spectroscopy (TGS), in which the stimulated emission induced by the injection of weak DUMP radiation is detected, was also applied to the C ′ → A 2 1 1 band system. The amplification of DUMP laser intensities at the exit of the ND 3 sample cell was successfully observed when the DUMP frequencies were resonant with the specific rovibronic lines in the C ′ → A band. The maximum gain was as large as 75. TGS spectra exhibited narrower line widths than the natural line widths estimated from the lifetimes of lower levels.

Pressure-induced rotational energy transfer in H 2CO [formula omitted]: Dipolar M-dependence with no single-collision elastic contribution

We have performed a series of measurements on H 2CO A ̃ 1A 2 v 4 = 1 single rotational levels using Transient Gain Spectroscopy (TGS), which was designed to provide detailed information on state-to-state population transfer and on the relaxation and transfer of rotational alignment. Measurements of the time dependence of the population of the directly populated J K a , K c = 1 0,1 level, and of the population that is collisionally transferred to the neighboring 0 0,0, 2 0,2, and 3 0,3 rotational levels, are performed with both parallel and perpendicular relative PUMP/PROBE polarizations. This procedure allows the rotational-state-resolved populations to be analyzed with a microscopic M-resolved kinetic model at a level of detail unprecedented for a polyatomic molecule. Our analysis is able to distinguish direct, single-collision dealignment from sequential processes that result in dealignment. Nonlinear fitting of these data with a number of kinetic models indicates that there is no detectable contribution from single-collision direct-elastic events to the observed dealignment signals, and that dealignment may be accurately modeled by sequential processes following electric-dipole a-dipole selection rules. Furthermore, state-to-state inelastic rates are found to partition into M J -resolved collisional transfer rates according to tensor opacity rank Λ = 1 (electric-dipole amplitudes), with no detectable contribution from Λ = 2 or Λ = 3. Tensor opacity rank Λ = 1 produces the greatest persistence of the M J quantum number consistent with collisions in an isotropic environment, and this high level of M J conservation is necessary to explain our data. An Energy-Corrected Sudden (ECS) scaling model, with collision duration τ c = 7.0 psec, and basis rate coefficient k 0←1 = 26.6(5) μsec −1 Torr −1 provides an excellent representation of the observed energy transfer, with no detectable contribution from higher order terms, or from terms deviating from ECS scaling.

Collisional relaxation of H2CO (Ã 1A2, v4=1, J K a ,K c=132,12) by He, Ar, Xe, and N2

The collision-induced deactivation of H2CO in a specific rotational level in its first electronically excited singlet state, Ã 1A2, v4=1, JKa,Kc=132,12, has been investigated with a series of colliders, M=He, Ar, Xe, and N2. The target level was populated via the transition Ã←X̃, 410, 132,12←133,11 using a pulsed dye laser. The subsequent relaxation was monitored using the technique of transient gain spectroscopy (TGS) via the transition Ã→X̃, 201301414, 132,12→131,13, which coincides with the line of an Ar+ laser at λ=488.0 nm. The experiments yielded the overall collisional depopulation rate constants (velocity averaged cross sections) for He, Ar, Xe, and N2, respectively: (5.25±0.20)×10−10 cm3/molecule s (39 Å2), (5.22±0.18)×10−10 cm3/molecule s (86 Å2), (5.37±0.29)×10−10 cm3/molecule s (106 Å2), (8.25±0.29)×10−10 cm3/molecule s (125 Å2), where the uncertainties correspond to the 2σ standard deviations.

Polarization-detected transient gain studies of relaxation processes in v4=1 Ã 1A2 formaldehyde-h2

Polarization-detected transient gain spectroscopy (PTGS), a time-resolved PUMP/PROBE scheme with single rotation–vibration level selectivity in both preparative and monitoring stages, has been used to study total depopulation and rotational state-to-state rates in the H2CO Ã 1A2 v4=1 vibrational level. For H2CO(Ã)/H2CO(X̃) collisions, single rovibronic level depopulation rates in the range 88±2 to 127±6 μs−1 Torr−1 (∼10 times faster than the hard sphere gas kinetic rate) were obtained from Stern–Volmer plots which were found to be linear over at least the 0.02–0.2 Torr pressure region. Rotational relaxation between the 50,5 and 60,6 eigenstates reflects a-dipole propensity rules with the sum of first order ΔJ=+1 and ΔJ=−1 processes accounting for well over 50% of the total removal cross section measured for a single rovibronic level. The agreement between collisional decay rates for the 132,12 eigenstate measured previously by the TGS technique and here by PTGS, 110±3 and 106±4 μs−1 Torr−1, respectively, suggests that elastic reorientation processes (ΔJ=ΔKa =ΔKc =0, ΔM≠0) occur with much less efficiency than inelastic energy transfer. Collisional depopulation from single rotational levels of H2CO Ã 1A2 v4=1 is a remarkably efficient process, but completely understandable in the small molecule, isolated binary collision limit. None of the multiexponential decay and nonlinear Stern–Volmer effects manifest in previous undispersed fluorescence studies remain when PTGS is used to populate and monitor a single rovibronic level.

Comment on Dennis Kavanagh's ‘how we vote now’

We have performed a series of measurements on H2CO Ã 1A2 v4 = 1 single rotational levels using Transient Gain Spectroscopy (TGS), which was designed to provide detailed information on state-to-state population transfer and on the relaxation and transfer of rotational alignment. Measurements of the time dependence of the population of the directly populated JKa,Kc = 10,1 level, and of the population that is collisionally transferred to the neighboring 00,0, 20,2, and 30,3 rotational levels, are performed with both parallel and perpendicular relative PUMP/PROBE polarizations. This procedure allows the rotational-state-resolved populations to be analyzed with a microscopic M-resolved kinetic model at a level of detail unprecedented for a polyatomic molecule. Our analysis is able to distinguish direct, single-collision dealignment from sequential processes that result in dealignment. Nonlinear fitting of these data with a number of kinetic models indicates that there is no detectable contribution from single-collision direct-elastic events to the observed dealignment signals, and that dealignment may be accurately modeled by sequential processes following electric-dipole a-dipole selection rules. Furthermore, state-to-state inelastic rates are found to partition into MJ-resolved collisional transfer rates according to tensor opacity rank Λ = 1 (electric-dipole amplitudes), with no detectable contribution from Λ = 2 or Λ = 3. Tensor opacity rank Λ = 1 produces the greatest persistence of the MJ quantum number consistent with collisions in an isotropic environment, and this high level of MJ conservation is necessary to explain our data. An Energy-Corrected Sudden (ECS) scaling model, with collision duration τc = 7.0 psec, and basis rate coefficient k0←1 = 26.6(5) μsec−1 Torr−1 provides an excellent representation of the observed energy transfer, with no detectable contribution from higher order terms, or from terms deviating from ECS scaling.

Rotational relaxation in the H2CO Ã 1A2 state by transient gain spectroscopy

A new, pulsed pump–cw probe technique, transient gain spectroscopy, has been used to measure the zero pressure lifetime (279±10 ns) and self-collision depopulation rate (110.5±3.1 μs−1 Torr−1) for the H2CO à 1A2 41 JKa,Kc =132,12 level as well as the 122,11←132,12(31±8 μs−1 Torr−1) and 142,13←132,12(40±6 μs−1 Torr−1) rotational relaxation rates. The dominant collisional process is rotational energy transfer, which, for the H2CO(Ã)+H2CO(X̃) process examined here, appears to follow dipole–dipole scaling and propensity rules. The 110.5 μs−1 Torr−1 depopulation rate in the à state agrees with the vibrationless X̃ state rate obtained from microwave power broadening scaled by the 1.46 D/2.33 D electric dipole moment ratio. The two strongest a-dipole rotational state-to-state rates account for more than 50% of the total removal rate from H2CO à 41 132,12.