Laser-induced Fluorescence Bands Research Articles

Laser-induced fluorescence and its effect on the damage resistance of fluoride-containing phosphate-based glasses

A series of multi-component fluoride-containing phosphate-based glasses prepared in a reducing atmosphere showed improved resistance to high-energy ultraviolet (UV) laser-induced damage and strong laser-induced fluorescence (LIF) within the glass bulk. The UV optical absorption, photoluminescence, and fluorescence decay properties of these glasses were investigated to explore the defect-related LIF mechanism and its underlying effect on the glasses’ laser-induced damage threshold (LIDT). Seven laser wavelengths ranging from 253 nm to 532 nm were used to excite the LIF, and two characteristic fluorescence bands peaking at approximately 414 nm and 780–800 nm occurred in all three types of glasses. The LIF band at 414 nm was attributed to PO3-EC defects in the second harmonic frequency (2ω) absorptive glass and third harmonic frequency (3ω) transparent glass, but Fe2+ ions in the fundamental frequency (1ω) absorptive glass. A later fluorescence band at 780–800 nm occurred due to POHC defects in the 2ω absorptive and 3ω transparent glasses and Fe3+ ions in the 1ω absorptive glass. A detailed study on the dynamic decay processes of two additional dominant fluorescence peaks at 450 nm and 780 nm under 266 nm excitation revealed the potential effect of LIF on LIDT improvement. The relatively longer LIF lifetime, higher LIF intensity, and larger LIF peak area corresponded with and contributed to a higher LIDT, especially in the 3ω transparent glass with a low UV absorption coefficient. This study provides strong evidence for the prior hypothesis between strong LIF and LIDT.

The electronic spectrum of the jet-cooled stibino (SbH2) free radical.

The Ã2A1-X̃2B1 electronic transition of the jet-cooled stibino (SbH2 and SbD2) free radical has been observed for the first time using laser induced fluorescence (LIF) detection. The radicals were produced by a pulsed electric discharge through a mixture of stibine (SbH3 or SbD3) in high pressure argon at the exit of a pulsed molecular beam valve. SbH2 exhibits only three LIF bands, assigned as 21 0, 00 0, and 20 1, with a fluorescence lifetime (τ), which decreases from ∼50 ns for 00 to <10 ns for 21. LIF transitions to the 00 (τ ∼ 2 µs), 21 (τ ∼ 400 ns), and 22 (τ ∼ 75 ns) upper vibronic states of SbD2 were also observed. High-resolution spectra exhibited large spin-rotation splittings and small resolved antimony hyperfine splittings due to a substantial Fermi contact interaction in the excited state. The experimentally determined rotational constants gave effective molecular structures of r0 ″ = 1.724(2) Å, θ0 ″ = 90.38(7)° and r0 ' = 1.693(6) Å, θ0 ' = 120.6(3)°. The ground state bending vibrational levels up to eight quanta (6404 cm-1) in SbH2 and 12 quanta (6853 cm-1) in SbD2 were measured from dispersed fluorescence spectra. All indications are that SbH2 undergoes a dissociative process at low vibrational energies in the excited electronic state.

Detection and characterization of the tin dihydride (SnH2 and SnD2) molecule in the gas phase.

The SnH2 and SnD2 molecules have been detected for the first time in the gas phase by laser-induced fluorescence (LIF) and emission spectroscopic techniques through the Ã1B1-X̃1A1 electronic transition. These reactive species were prepared in a pulsed electric discharge jet using (CH3)4Sn or SnH4/SnD4 precursors diluted in high pressure argon. Transitions to the electronic excited state of the jet-cooled molecules were probed with LIF, and the ground state energy levels were measured from single rovibronic level emission spectra. The LIF spectrum of SnD2 afforded sufficient rotational structure to determine the ground and excited state geometries: r0″ = 1.768 Å, θ0″ = 91.0°, r0' = 1.729 Å, θ0' = 122.9°. All of the observed LIF bands show evidence of a rotational-level-dependent predissociation process which rapidly decreases the fluorescence yield and lifetime with increasing rotational angular momentum in each excited vibronic level. This behavior is analogous to that observed in SiH2 and GeH2 and is suggested to lead to the formation of ground state tin atoms and hydrogen molecules.

Dispersed Fluorescence Spectroscopy of Jet-Cooled 2-, 3-, and 4-Methylcyclohexoxy Radicals.

Vibrational structures of the nearly degenerate X̃ and à states of the 2-, 3-, and 4-methylcyclohexoxy (MCHO) radicals were studied by jet-cooled dispersed fluorescence (DF) spectroscopy. The observed transitions were assigned on the basis of vibrational frequencies and Franck-Condon factors predicted by quantum chemical calculations. Intensities of vibronic transitions in the DF spectra are dependent on the laser-induced fluorescence (LIF) bands pumped in the experiment, which can be explained by the difference in geometry and symmetry between the lower X̃/à states and the highly excited B̃ state. All three studied isomers of MCHO have close-lying X̃ and à states although their energy separations are affected by the position of the methyl group. It is suggested by quantum chemical calculations that the lowest-energy conformers of all three isomers have the half-filled orbital oriented perpendicular to the OCH plane, which is consistent with the observed relative intensities of the B̃ → X̃ and B̃ → à origin bands. When the origin and the CO-stretch bands of the B̃ ← X̃ LIF excitation spectra were pumped, the DF spectra were dominated by CO-stretch progressions. When non-CO-stretch vibrational levels of the B̃ state were pumped, progressions of CO-stretch modes combined with the pumped vibrational mode were observed. Excited-state vibrational population relaxation from the CO stretch level to the vibrational ground level and from combination levels of the CO stretch mode and other vibrational modes to the non-CO stretch modes was observed. Analysis of the DF spectra confirms the previous conclusion that all strong LIF bands observed under jet-cooled conditions belong to a single conformer of each positional isomer (Lin et al. RSC Adv. 2012, 2, 583-589).

BH2 revisited: New, extensive measurements of laser-induced fluorescence transitions and ab initio calculations of near-spectroscopic accuracy.

The spectroscopy of gas phase BH2 has not been explored experimentally since the pioneering study of Herzberg and Johns in 1967. In the present work, laser-induced fluorescence (LIF) spectra of the Ã(2)B1(Πu)-X̃ (2)A1 band system of (11)BH2, (10)BH2, (11)BD2, and (10)BD2 have been observed for the first time. The free radicals were "synthesized" by an electric discharge through a precursor mixture of 0.5% diborane (B2H6 or B2D6) in high pressure argon at the exit of a pulsed valve. A total of 67 LIF bands have been measured and rotationally analyzed, 62 of them previously unobserved. These include transitions to a wide variety of excited state bending levels, to several stretch-bend combination levels, and to three ground state levels which gain intensity through Renner-Teller coupling to nearby excited state levels. As an aid to vibronic assignment of the spectra, very high level hybrid ab initio potential energy surfaces were built starting from the coupled cluster singles and doubles with perturbative triples (CCSD(T))/aug-cc-pV5Z level of theory for this seven-electron system. In an effort to obtain the highest possible accuracy, the potentials were corrected for core correlation, extrapolation to the complete basis set limit, electron correlation beyond CCSD(T), and diagonal Born-Oppenheimer effects. The spin-rovibronic states of the various isotopologues of BH2 were calculated for energies up to 22 000 cm(-1) above the X̃ (000) level without any empirical adjustment of the potentials or fitting to experimental data. The agreement with the new LIF data is excellent, approaching near-spectroscopic accuracy (a few cm(-1)) and has allowed us to understand the complicated spin-rovibronic energy level structure even in the region of strong Renner-Teller resonances.

An experimental and ab initio study of the electronic spectrum of the jet-cooled F2BO free radical.

We have studied the B̃ (2)A1-X̃ (2)B2 laser-induced fluorescence (LIF) spectrum of the jet-cooled F2BO radical for the first time. The transition consists of a strong 0(0)(0)band at 446.5 nm and eight weak sequence bands to shorter wavelengths. Single vibronic level emission spectra obtained by laser excitation of individual levels of the B̃ state exhibit two electronic transitions: a very weak, sparse B̃-X̃ band system in the 450-500 nm region and a stronger, more extensive set of B̃ (2)A1-à (2)B1 bands in the 580-650 nm region. We have also performed a series of high level ab initio calculations to predict the electronic energies, molecular structures, vibrational frequencies, and rotational and spin-rotation constants in the X̃ (2)B2, à (2)B1 and B̃ (2)A1 electronic states as an aid to the analysis of the experimental data. The theoretical results have been used as input for simulations of the rotationally resolved B̃ (2)A1-X̃ (2)B2 0(0)(0) LIF band and Franck-Condon profiles of the LIF and single vibronic level emission spectra. The agreement between the simulations obtained with purely ab initio parameters and the experimental spectra validates the geometries calculated for the ground and excited states and the conclusion that the radical has C2v symmetry in the X̃, Ã, and B̃ states. The spectra provide considerable new information about the vibrational energy levels of the X̃ and à states, but very little for the B̃ state, due to the very restrictive Franck-Condon factors in the LIF spectra.

Spectroscopic and Theoretical Investigations of ThS and ThS+

Gas-phase ThS has been produced via the reaction of laser ablated Th with H2S. Rotationally resolved electronic spectra were recorded by laser-induced fluorescence (LIF) over the range 17500-24000 cm(-1). Resonance-enhanced multiphoton ionization was used in conjunction with a time-of-flight mass spectrometer to confirm the assignments of nine LIF bands to ThS. Using excitation of a ThS band centered at 22118 cm(-1), a dispersed fluorescence spectrum revealed a vibrational progression of the X(1)Σ(+) ground electronic state and the term energies of two low-lying excited states ((3)Δ1 and (3)Δ2). Two-color photoionization spectroscopy was used to study ThS(+). An accurate ionization energy for ThS was obtained (54425(3) cm(-1)); ThS(+) vibronic term energies up to v = 7 in the X(2)Σ(+) ground state and v = 3 in the (2)Δ3/2 first excited state were recorded. High-level electronic structure calculations, with inclusion of the spin-orbit interactions yielded predictions that were in good agreement with the experimental data for ThS and ThS(+). The spectroscopic properties of ThS/ThS(+) are compared with those of the valence isoelectronic pairs ThO/ThO(+), HfO/HfO(+), and HfS/HfS(+).

Laser-induced fluorescence of fused silica irradiated by ArF excimer laser

Laser-induced fluorescence (LIF) of high-purity fused silica irradiated by ArF excimer laser is studied experimentally. LIF bands of the fused silica centered at 281 nm, 478 nm, and 650 nm are observed simultaneously. Furthermore, the angular distribution of the three fluorescence peaks is examined. Microscopic image of the laser modified fused silica indicates that scattering of the generated fluorescence by laser-induced damage sites is the main reason for the angular distribution of LIF signals. Finally, the dependence of LIF signals intensities of the fused silica on laser power densities is presented. LIF signals show a squared power density dependence, which indicates that laser-induced defects are formed mainly via two-photon absorption processes.

Laser induced fluorescence spectroscopy of the jet-cooled carbon dioxide cation (C12O2+ and C13O2+)

The A (2)Pi(u)-X (2)Pi(g) band systems of jet-cooled (12)CO(2)(+) and (13)CO(2)(+) have been recorded by laser-induced fluorescence (LIF) techniques. Very intense, vibrationally cold expansions of these cations have been obtained using a pulsed electric discharge jet with a precursor mixture of carbon dioxide or (13)C labeled CO(2) in high pressure argon. The LIF bands have been partially rotationally analyzed to obtain band origins which yielded an accurate measure of the excited state vibronic energy levels. The energy levels of both isotopologues were fitted with a Renner-Teller model that included spin-orbit coupling, Fermi resonance and anharmonic terms. Single vibronic level emission spectra were also recorded for the (13)CO(2)(+) ion and the ground state energy levels fitted using the same Renner-Teller model. The isotope relations have been used to test the validity of the derived parameters. The results give a through description of the vibronic energy levels in the ground and first excited electronic states of the carbon dioxide cation.

Electronic spectroscopy of the jet-cooled arsenic dicarbide (C2As) free radical

The (2)Delta(r)-X (2)Pi(r) band system of the jet-cooled arsenic dicarbide (C(2)As) free radical has been recorded by laser-induced fluorescence (LIF) techniques in the 685-588 nm region. The radical was produced in a pulsed electric discharge jet using a precursor mixture of AsCl(3) vapor and methane in high pressure argon. A series of weak bands involving all three excited state vibrations was observed for both (12)C(2)As and (13)C(2)As. High-resolution spectra of the (2)Pi(12) component of the 0(0)(0) bands of both isotopomers were rotationally analyzed, leading to the conclusion that the upper state is (2)Delta with a small spin-orbit splitting (A = 2.78 cm(-1)). Ground and excited state molecular structures of r(0)(")(CC,ab initio) = 1.2933 A, r(0)(")(CAs) = 1.734(4) A and r(0)(')(CC,ab initio) = 1.2276 A, r(0)(')(CAs) = 1.830(3) A were derived from the B values and our density functional predictions of the C-C bond lengths. Single vibronic level emission spectra were recorded for many of the LIF bands and these were used to obtain the ground state vibrational frequencies and spin-orbit splittings. These data were satisfactorily fitted to a Renner-Teller model which gave (12)C(2)As parameters of epsilon = 0.695(8), omega(1) = 1704.8(20) cm(-1), omega(2) = 161.6(8) cm(-1), omega(3) = 663.6(12) cm(-1), and a spin-orbit constant A = 857.7(11) cm(-1).

Laser induced fluorescence bands in the FT-Raman spectra of bioceramics

A new fluorescence band at about 1930 cm −1 has been observed in the FT-Raman spectra of some bioceramics. While a fluorescence band at about 760 cm −1 has been already reported in the FT-Raman spectra of hydroxyapatite and related calcium phosphates, this new band is observed for the first time. This strong band is totally absent in the anti-Stokes side of the FT-Raman spectra and therefore has been assigned as a laser induced fluorescence band. It has been concluded that in recording FT-Raman spectra of minerals, all precautions about fluorescence bands should be considered, otherwise the FT-Raman spectra may well be misinterpreted.

Renner–Teller vibronic analysis for a tetra-atomic molecule. II. The ground state of the HCCS free radical

We have studied the tetra-atomic Renner-Teller effect in the ground state of the jet-cooled HCCS and DCCS free radicals. The reactive intermediates were produced in a pulsed discharge jet using thiophene or deuterated thiophene precursors and the ground-state vibronic energies were measured using the single vibronic level emission spectroscopy technique. In HCCS, emission and fluorescence depletion experiments proved that some previously unassigned laser-induced fluorescence (LIF) features were anomalous 2Delta32-2Pi32 transitions, which gave information on several ground-state 2Delta32 levels. In DCCS, rotational mixing of the excited state v'=0(2Pi32) level with the 4(1)(1 2Sigma12) level allowed several ground-state 2Sigma levels to be observed in emission. In addition, we have shown that some previously unassigned weak LIF bands of DCCS are anomalous 2Pi12-2Pi32 transitions which have allowed us to accurately measure the ground-state spin-orbit splitting (259.0 cm(-1)) for the first time. All of the expected ground-state 2Pi32 levels and the majority of the 2Pi12 levels up to 1700 cm(-1) have been observed for both isotopomers. The data have been fitted by an effective Hamiltonian matrix treatment that included Renner-Teller, spin-orbit coupling, anharmonicity, and Fermi resonance effects for the two bending vibrations and the CS stretching mode. This has allowed an unusually complete description of the orbital angular momentum coupling effects, including the determination of the epsilon4, epsilon5, and epsilon45 terms for the first time. Subtle effects in both the LIF and emission spectra signal the presence of Sears resonances in both the ground and excited states of these radicals.

Remote Raman and fluorescence studies of mineral samples

In the present study, we investigated remote laser-induced fluorescence (LIF), at a distance of 4.8 m, of a variety of natural minerals and rocks, and Hawaiian Ti ( Cordyline terminalis) plant leaves. These minerals included calcite cleavage, calcite onex and calcite travertine, gypsum, fluorapatite, Dover flint and chalk, chalcedony and nephelene syenite, and rubies containing rock. Pulsed laser excitation of the samples at 355 and 266 nm often resulted in strong fluorescence. The LIF bands in the violet-blue region at ∼413 and ∼437 nm were observed only in the spectrum of calcite cleavage. The green LIF bands with band maxima in the narrow range of ∼501–504 nm were observed in the spectra of all the minerals with the exception of the nephelene syenite and ruby rocks. The LIF red bands were observed in the range ∼685–711 nm in all samples. Excitation with 532 nm wavelength laser gave broad but relatively low fluorescence background in the low-frequency region of the Raman spectra of these minerals. One microsecond signal gating was effective in removing nearly all background fluorescence (with peak at ∼610 nm) from calcite cleavage Raman spectra, indicating that the fluorescence was probably from long-lifetime inorganic phosphorescence.

Laser-induced fluorescence and hole-burning spectra of functional dyes in supersonic jets: spectra of a merocyanine dye

In LIF (laser-induced fluorescence) excitation and hole-burning spectra of a merocyanine dye, EtMD, in a supersonic jet, bands due to one of the four possible isomers were observed. Only lower-frequency vibronic bands were observed in the LIF-excitation spectra, while broad and deep dips were observed in the higher energy region of the hole-burning spectra where no LIF bands were observed. The LIF bands were found to consist of two transition systems, which are presumably due to the molecules in the ground and low-lying excited vibrational states. Possible relaxation processes of EtMD in the excited electronic state were discussed.

Radiative electronic transitions of rare gas molecular ions in cryomatrices

Abstract Intrinsic ionic centers in solid Xe were produced by (i) ionization in situ by vacuum ultraviolet (VUV) light and (ii) deposition of ion-containing gas on cooled substrate. Laser-induced fluorescence (LIF) from the samples was studied in the range of transitions between the ground state of ionic centers and the lower excited states related to the 2 P 1/2 , 2 P 3/2 limits of Xe ions. The influence of the lattice defects which act as traps for charge carriers was examined and conditions of the stability of ionic centers were discussed. The coincidence of the LIF band observed with the samples grown from discharge with that observed under band-to-band excitation was found. It suggests that the electronic structure of self-trapped holes (STHs) is identical with that of intrinsic ionic centers formed in solid Xe after deposition of ion-containing gas (Xe2+).

Laser-induced fluorescence spectroscopy of the ketenyl radical

We report the first laser-induced fluorescence (LIF) excitation spectrum of the ketenyl radical, HCCO, which is produced by the 193 nm photolysis of ketene in a free jet expansion. A series of vibronic bands in the B̃ 2Π–X̃ 2A′′ system are observed. The LIF band positions and rotational structures are in excellent agreement with those of a recent photofragment yield (PFY) spectrum [D. L. Osborn et al., J. Chem. Phys. (to be published)] from the origin at 33 424 cm−1 to 35 100 cm−1 . At higher energies the LIF spectrum breaks off sharply due to a rapid increase in the rate of predissociation. The lifetime of the vibrationless level of the B̃ state is less than 1 ns; the estimated quantum yield of fluorescence is ∼10−3.

Laser-induced fluorescence spectroscopy of CCH (X̃ 2Σ+) in vibrationally excited levels up to 4500 cm−1

Vibrationally excited levels of the CCH radical in the X̃ 2Σ+ state up to 4500 cm−1 above the ground vibrational level have been studied both at high pressure and in a supersonic jet by laser-induced fluorescence (LIF). The CCH radical was produced by photolyzing acetylene with 193 nm laser light. Nineteen UV bands are rotationally analyzed and assigned as transitions of parallel type from X̃(0,υl2,υ3), (υ2=0–10, υ3=0–2, l=0–3) to four K sublevels (K=0–3) of a common vibrational level, T, of an upper electronic state most likely B̃ 2A′. Among them, bands at 37 010 and 36 075 cm−1 were previously reported [J. Chem. Phys. 98, 6690 (1993)]; an improved spin–orbit constant of X̃(0,71,0) was determined in this work. Vibrational energies of the l=0 and 1 levels were determined in a simultaneous analysis of UV and IR bands. Assignment of a vibrational level T as the common upper state of the observed LIF bands permits us to determine spectroscopic parameters of the l=2 and 3 levels of the X̃ state, which are reported for the first time. The geometry of level T at 39 157.41 cm−1 is accordingly deduced and agrees with the ab initio prediction of the B̃ state. Our vibrational assignments of the X̃ state, particularly those of bending progressions, are supported by observations of the l=2 and 3 levels.

Symmetry demand in polar excited state formation of 9,9′-bianthryl in clusters with some ketones

Laser-induced fluorescence (LIF) and dispersed fluorescence (DF) spectra have been studied for clusters of 9,9′-bianthryl (BA) with acetone, diethyl ketone, and methyl–ethyl ketone. Two different kinds of clusters have been observed in clusters containing the symmetric ketones. One shows broad LIF and DF spectra which are characteristic to the polar excited state of BA. The other shows sharp LIF bands and short lifetime which indicate the absence of the electron transfer.