Laser-induced Dispersed Fluorescence Research Articles

Extending the Large Molecule Limit: The Role of Fermi Resonance in Developing a Quantum Functional Group.

Polyatomic molecules equipped with optical cycling centers (OCCs), enabling continuous photon scattering during optical excitation, are exciting candidates for advancing quantum information science. However, as these molecules grow in size and complexity, the interplay of complex vibronic couplings on optical cycling becomes a critical but relatively unexplored consideration. Here, we present an extensive exploration of Fermi resonances in large-scale OCC-containing molecules using high-resolution dispersed laser-induced fluorescence and excitation spectroscopy. These resonances manifest as vibrational coupling leading to intensity borrowing by combination bands near optically active harmonic bands, which require additional repumping lasers for effective optical cycling. To mitigate these effects, we explore altering the vibrational energy level spacing through substitutions on the phenyl ring or changes in the OCC itself. While the complete elimination of vibrational coupling in complex molecules remains challenging, our findings highlight significant mitigation possibilities, opening new avenues for optimizing optical cycling in large polyatomic molecules.

Vibrationally Resolved Absorption, Fluorescence, and Preresonance Raman Spectroscopy of Isolated Pyronin Y Cation at 5 K.

Exploring how charge-changing affects the photoluminescence of small organic dyes presents challenges. Here, helium tagging photodissociation (PD) action spectroscopy in the gas phase and dispersed laser-induced fluorescence (DF) spectroscopy in the solid Ne matrix are used to compare the intrinsic photophysical properties of pyronin Y cation [PY]+ and its one electron-reduced neutral radical [PY]• at 5 K. Whereas the cation shows efficient visible photoluminescence, no emission from the neutral, in line with theoretical predictions, was detected. B3LYP/aug-cc-pVDZ calculations based on the TD-DFT/FCHT method allow for unambiguous assignment of recorded vibrationally resolved absorption and emission spectra. Surprisingly, our experimental sensitivity was high enough to also observe electronic preresonance Raman (ePR-Raman) spectra of [PY]+, with a significant efficiency factor (EF). These characteristics of the [PY]•/[PY]+ pair suggest that appropriately functionalized derivatives may open new perspectives in the area of in vivo bioimagining microscopy and find applications in various sophisticated stimulated-Raman spectroscopies.

Electronic Spectroscopy of Jet-Cooled NdO

Chemi-ionization reactions of the type M + O → MO+ + e- (M = Nd or Sm) are currently being investigated as a method to artificially increase the electron density in the ionosphere for control of micro- and radio wave propagation. Experiments involving the release of atomic Nd into the upper atmosphere have resulted in the production of a cloud that, on excitation by solar radiation, emits green light. It has been assumed that NdO was the carrier of this emission, but the existing spectroscopic data needed for this attribution is lacking. While the electronic spectrum of NdO has been well-characterized at wavelengths greater than 590 nm, relatively little spectroscopic data exist for emission wavelengths in the blue-green spectral range. In this study, spectra for jet-cooled NdO were recorded over the range 15,500-21,000 cm-1. Rotationally resolved laser induced fluorescence and vibronically resolved dispersed laser-induced fluorescence spectra were recorded, and nine new electronically excited states were identified. The data indicate that the electronic spectrum of NdO has relatively few allowed transitions in the green spectral range, casting doubt on the assignment of the Nd high-altitude release cloud green emission to NdO.

Laser Spectroscopy of Aromatic Molecules with Optical Cycling Centers: Strontium(I) Phenoxides.

We report the production and spectroscopic characterization of strontium(I) phenoxide (SrOC6H5 or SrOPh) and variants featuring electron-withdrawing groups designed to suppress vibrational excitation during spontaneous emission from the electronically excited state. Optical cycling closure of these species, which is the decoupling of the vibrational state changes from spontaneous optical decay, is found by dispersed laser-induced fluorescence spectroscopy to be high, in accordance with theoretical predictions. A high-resolution, rotationally resolved laser excitation spectrum is recorded for SrOPh, allowing the estimation of spectroscopic constants and identification of candidate optical cycling transitions for future work. The results confirm the promise of strontium phenoxides for laser cooling and quantum state detection at the single-molecule level.

Electronic Spectroscopy of SmO in the 645 to 670 nm Range.

The associative ionization reaction Sm + O → SmO+ + e- is being investigated as an electron source that could transiently modify high-altitude electron densities via Sm vapor release. Electronic spectra have been obtained from tests where sounding rockets released Sm vapor, but the interpretation of these results has been hampered by the limited laboratory spectral data available for both SmO and SmO+. The present study extends the spectroscopic characterization of SmO in the 645-670 nm range, where the field data show the most prominent molecular emission features. Rotationally resolved excitation spectra, dispersed laser-induced fluorescence spectra, and fluorescence decay lifetimes are reported. The results are consistent with the assignment of a subset of the red-region bands to configurational transitions of the form Sm2+(4f56s)O2- ↔ Sm2+(4f55d)O2-. Analysis of the excited state hyperfine structure supports this configurational description.

Laser induced fluorescence spectra of the [formula omitted][formula omitted] – [formula omitted][formula omitted] transition of jet cooled [formula omitted]NO3 and [formula omitted]NO3 I: [formula omitted] progressions in the ground [formula omitted][formula omitted] state

14NO3 and 15NO3 were generated in a supersonic free jet, and their laser excitation spectra and dispersed laser induced fluorescence (LIF) spectra of the B∼2E′ – X∼2A2′ transitions from the vibrationless B∼2E′ state were observed. Because of the cooling of the gas in the supersonic jet, the observed spectra are with resolution an order of magnitude higher than those previously reported and allowed us to resolve, for example, the splitting between the a1′ and e′ components of the 2ν4 state. The spectra of 14NO3 and 15NO3 are similar but small isotope shift in the latter allowed us to separate the ν1 band from the 3ν4 (a1′) band which were overlapped in the former.The high resolution dispersed LIF spectra revealed panoramic views of the vibrational level structures in the X∼2A2′ ground electronic states of the two isotopomers. Although the accuracies of the measurement of energy levels are much less than those obtained from the direct high resolution infrared spectroscopy, the obtained patterns are more complete because totally symmetric (a1′) vibrations such as ν1,2ν2,2ν1,ν1 + 2ν2 etc. are also observed. Such clear and complete spectra have revealed remarkable ν4 progressions with detailed vibrational structures in the ground X∼2A2′ state. Near complete progressions of nν4 overtone bands, nν4 with n = 1 ∼ 5, ν1 + nν4 bands with n = 1 ∼ 4, some 2ν2 + nν4 and ν3 + nν4 have been assigned leaving few lines unassigned. There is no evidence of anomaly in the degenerate ν4 bending progression of the B∼2E′ – X∼2A2′ electronic transition of NO3.

Radiative force from optical cycling on magnesium monofluoride

We demonstrate radiative force deflection by using an optical cycling of the $X{\phantom{\rule{0.16em}{0ex}}}^{2}{\mathrm{\ensuremath{\Sigma}}}_{1/2}^{+}\ensuremath{-}A{\phantom{\rule{0.16em}{0ex}}}^{2}{\mathrm{\ensuremath{\Pi}}}_{1/2}$ electronic transition in diatomic molecule magnesium monofluoride (MgF). For the (0,0) and (0,1) bands of the electronic transition, the Franck-Condon factors---obtained by the dispersed laser-induced fluorescence spectrum---are ${f}_{00}=0.972$ and ${f}_{01}=0.028$, which suggests that the vibrational branching is quasiclosed. Furthermore, the dark Zeeman sublevels are destabilized by applying an external magnetic field. The cycling fluorescence is clearly observed to reveal that the molecular beam is deflected by 4.9 mm via scattering $\ensuremath{\sim}200$ photons per molecule. This work is encouraging toward laser cooling and magneto-optical trapping of MgF molecules.

Spectroscopic Study of the B 1Π State of NaH.

In this study, the B1Π excited state of NaH has been experimentally studied for the first time. Pulsed laser-induced fluorescence excitation spectroscopy was used to investigate the B1Π electronic state of NaH. A total of 48 ro-vibronic transitions were observed, including within the B–X (0–0) and B–X (0–1) transition bands. Only one B-state vibrational level was identified, and a series of PQR lines, with eight e-parity and eight f-parity sublevels (v′ = 0, J′ = 1–8), were assigned. The level assignment was supported by a comparison of the experimental line positions with the ab initio calculations, the dispersed laser-induced fluorescence spectrum of the NaH B1Π → X1∑+ emission, and the V-type optical–optical double resonance spectra. The Dunham-type coefficients, the mean internuclear distance, the harmonic vibrational frequency ω, and the dissociation energies D0 and De of the B1Π state were determined.

Perturbations of the A′1Π and C1Σ+ states of CaO

Laser induced fluorescence spectra for jet-cooled CaO have been used to examine the higher vibrational levels of the A′1Π(Xσπ−1) state (v = 10–17) and the v = 6–8 vibrational levels of the C1Σ+(Bσσ−1) state. A perturbation of v = 17 of the A′ state was evident, caused by a spin-orbit mediated interaction with the b3Σ+(Ω = 1)(Xσσ−1) v = 14 level. Ro-vibrational constants for the A′ state were obtained by fitting to the data for nominally unperturbed levels. The average radiative lifetime for the A′1Π(Xσπ−1) state v = 10–17 levels was 2.6 μs, with no measurable dependence on v. This was consistent with the theoretical prediction of a relatively small transition dipole moment for the A′1Π–X1Σ+ band system.A homogeneous perturbation of the C1Σ+(Bσσ−1) v = 7 level was examined by means of dispersed laser induced fluorescence (DLIF) and radiative lifetime measurements. The DLIF spectra clearly demonstrate that the perturbing Ω = 0+ state is a component of a triplet state. Intense transitions from this state to the a3Π(0+)(Xσπ−1) state were observed. Configurational arguments are presented that support assignment of the perturbing state as the v = 12 vibrational level of the previously unobserved g3Π(0+)(Bσπ−1) state.

Branching Ratios, Radiative Lifetimes, and Transition Dipole Moments for YbOH.

Medium resolution (Δν̃ ∼ 3 GHz) laser-induced fluorescence (LIF) excitation spectra of a rotationally cold sample of YbOH in the 17300-17950 cm-1 range have been recorded using two-dimensional (excitation and dispersed fluorescence) spectroscopy. High resolution (Δλ ∼ 0.65 nm) dispersed laser-induced fluorescence (DLIF) spectra and radiative decay curves of numerous bands detected in the medium resolution LIF excitation spectra were recorded. The vibronic energy levels of the X̃2Σ+ state were predicted using a discrete variable representation approach and compared with observations. The radiative decay curves were analyzed to produce fluorescence lifetimes. DLIF spectra resulting from high resolution (Δν̃ < 10 MHz) LIF excitation of individual low-rotational lines in the Ã2Π1/2(0,0,0)-X̃2Σ+(0,0,0), Ã2Π1/2(1,0,0)-X̃2Σ+(0,0,0), and [17.73]Ω = 0.5(0,0,0)-X̃2Σ+(0,0,0) bands were also recorded. The DLIF spectra were analyzed to determine branching ratios which were combined with radiative lifetimes to obtain transition dipole moments. The implications for laser cooling and trapping of YbOH are discussed.

Vibrational relaxation of S2(a1Δg, υ = 1–9) by collisions with He

Vibrationally excited S2(a1Δg, υ=1–10) was generated by the S(1D)+OCS reaction and detected with dispersed laser-induced fluorescence (LIF) via the f1Δu–a1Δg transition. The time profiles of the vibrational levels of interest were recorded at varying pressures of He. A kinetic analysis made by the integrated profiles method has given the rate coefficients for vibrational relaxation of S2(a1Δg,υ=1–9) by collisions with He. The energy gap law nicely correlates the probabilities of vibrational energy transfer per collision from S2(a1Δg), O2(X3Σg-), NO(X2Π), and CO(X1Σ+) to He.

Jet-cooled laser-induced dispersed fluorescence spectroscopy of TaN: Observation of a3Δ and A1Δ states

Laser-induced dispersed fluorescence spectra of TaN molecules, produced in a free-jet apparatus, have been studied. Two spin components of the lowest-lying a3Δ state along with their vibrational structure have been observed. The A1Δ state, which was predicted earlier by ab initio calculation has also been observed. The X1Σ+ ground state vibrational progression up to v=9 has been recorded. The experimentally determined term energies and vibrational constants at equilibrium for the ground and a3Δ states are in fairly good agreement with the ab initio values reported earlier.

Kinetics and dynamics on the formation of S2(X3Σg–,a1Δg) in the S(1D) + OCS reaction.

The reaction of electronically excited sulfur S((1)D) with OCS has exothermic channels generating S(2) in two electronic states X(3)Σ(g)– and a(1)Δ(g). The a(1)Δ(g) state is correlated directly to the reactants via the spin-allowed singlet surface; the X(3)Σ(g)– state, on the other hand, is a product of the spin-forbidden channel. There has been no report on kinetic evidence for the simultaneous generation of the two electronic states, although the two electronic states have been detected so far. The previous studies showed that little energy was released into rotation or vibration of the S(2) products despite large heats of reactions (228 and 175 kJ mol(-1) for generation of X(3)Σ(g)– and a(1)Δ(g), respectively). In the present study, S((1)D) was generated by the photolysis of OCS at 248 nm in a buffer He at 298 K, and the resulting two electronic states of S(2) (X(3)Σ(g)– and a(1)Δ(g)) were detected with dispersed laser-induced fluorescence (LIF) via the B(3)Σ(u)(-)–X(3)Σ(g)– and f(1)Δ(u)-a(1)Δ(g) transitions, respectively. Not only excitation but also dispersed fluorescence spectra made it possible to find a single rotational line of the vibrational level of interest. The time-resolved LIF intensities of the initial growth of the X(3)Σ(g)– and a(1)Δ(g) states showed identical OCS pressure dependences, giving the overall rate coefficient of the S(1D) + OCS reaction to be [3.2 ± 0.2(2σ)] × 10(-10) cm3 molecule(-1) s(-1). The simultaneous generation of the two electronic states indicates that the intersystem crossing plays a role in opening the spin-forbidden channel. As for the reaction dynamics, vibrational levels up to v = 19 of X(3)Σ(g)– and 11 of a(1)Δ(g) have been detected, which is distinctly different from the previous studies. The reaction mechanism has been discussed on the basis of the potential energies reported so far.

Jet-cooled laser-induced dispersed fluorescence spectroscopy of NiC: Observation of low-lying Ω = 0+ state

Laser-induced dispersed fluorescence spectra of 58Ni12C molecules, produced in a free-jet apparatus, have been studied. A new low-lying Ω=0+ state has been observed at Te=5178 (6)cm−1. Based on previous ab initio calculations this state is plausibly assigned as 0+ spin–orbit component of the first excited 3Π state. The term energies of vibrational levels up to v=10 for X1Σ+ ground and v=3 for Ω=0+ states have been determined. The harmonic and anharmonic wavenumbers respectively equal to 833 (4) and 6.7 (13)cm−1 for Ω=0+ state have been measured.

Experimental Determination of the Vibrational Constants of FeS(X5Δ) by Dispersed Fluorescence Spectroscopy

Based on previous laser-induced fluorescence excitation spectroscopy work, the vibrational constants of neutral FeS in the X5Δ electronic state were obtained by directly mapping the ground-state vibrational levels up to v=3 using conventional laser-induced dispersed fluorescence spectroscopy. The vibrational frequency of FeS(X5Δ) (518±5 cm−1) agrees well with that reported in a recent PES measurement (520±30 cm−1) [J. Phys. Chem. A 107, 2821 (2003)] which is the only one prior experimental vibrational frequency value for the 5Δ state of FeS. Careful comparisons of our experimental results and those documented in the literature (mainly from theoretical predictions) suggest that the ground state of FeS is 5Δ state.

Jet-cooled laser-induced dispersed fluorescence spectroscopy of ScN: Observation of a3Σ+ state

Laser-induced dispersed fluorescence spectra of ScN molecules, produced in a free-jet apparatus, have been studied. Low-energy first excited state, a3Σ+ predicted by ab initio calculations, has been observed for the first time. The relative term energies of the vibrational levels of the X1Σ+ ground, a3Σ+, and A1Σ+ states have been determined. The harmonic wavenumbers equal to 787.7(3) and 1044.8(1)cm−1, respectively for the X1Σ+ and A1Σ+ states and ΔG1/2=888(11)cm−1 for the a3Σ+ state have been measured. RKR potential energy curves of the X1Σ+ ground as well as a3Σ+ and A1Σ+ low-energy excited states have been constructed.

Fine structure-resolved rotational energy transfer of SH (A2Σ+, v′ = 0) state by collisions with Ar

Rotational energy transfer (RET) by Ar collisions within the v' = 0 level of the SH A(2)Sigma(+) state is probed using a laser-induced dispersed fluorescence technique, following photodissociation of H(2)S at 248 nm. The Ar pressure is adjusted appropriately to allow for significant observation of the single-collision induced RET process. The spin-resolved and spin-averaged rate constants are then evaluated with the aid of a kinetic model under single-collision conditions. The theoretical counterparts are calculated using a quantum scattering method, in which a newly fitted potential energy function is based on ab initio potential energy surface reported previously. The experimental and theoretical kinetic data are essentially consistent in the trend of N and DeltaN dependence. Several propensity rules are found in the RET collisions. For instance, for DeltaN = 1, 2, and 3, the rate constants decrease with increasing N or DeltaN. Given a fixed DeltaN, the rate constants of the same initial N in the downward transition appear to be larger than those in the upward transitions. In DeltaN = 0, the F(2)--> F(1) transitions prevail over the F(1)--> F(2) transitions (F(1) = N + 1/2, F(2) = N - 1/2), whereas in DeltaN not equal 0, the fine-structure-conserving collisions are more favored than the fine-structure-changing collisions. The principle of microscopic reversibility is also examined for both experimental and theoretical kinetic data, showing that translational energies of the RET collisions are close to thermal equilibrium at room temperature. The propensity rules may be rationalized according to this principle.

Spectroscopic Investigation of the Multiphoton Photolysis Reactions of Bromomethanes (CHBr3, CHBr2Cl, CHBrCl2, and CH2Br2) at Near-Ultraviolet Wavelengths

Nascent emission and laser-induced dispersed fluorescence spectra of products or intermediates from the multiphoton photolysis reaction of bromomethanes (CHBr(3), CHBr(2)Cl, CHBrCl(2), and CH(2)Br(2)) at 266 nm were recorded in a slow flow cell. Electronically excited species including CH (A(2)Delta, B(2)Sigma(-), and C(2)Sigma(+)), C(2) (d(3)Pi(g)), and atomic Br ((4)D(J) and (4)P(J)) were observed in the nascent emission spectra. Free radicals such CHBr or CHCl were also successfully found using laser-induced dispersed fluorescence spectroscopy. The reactive intermediate, CHBr, was seen only in the photolysis of CHBr(3), whereas CHCl was only discovered when the precursor was CHBr(2)Cl or CHBrCl(2). More experiments including the power dependence and temporal waveform measurements were conducted. The present study reports the first direct measurements of the intermediate products in the multiphoton photodissociation reaction of these bromomethanes at 266 nm. Nascent emission spectra following the photolysis at longer near-ultraviolet wavelengths (280 and 355 nm) were also acquired. On the bassis of these results, the multiphoton photodissociation mechanism of these bromomethanes at 266 nm can be confirmed.

The electronic transition moment function for the [formula omitted] system of TiO

Several vibronic bands associated with v ′=0 and 1 for the C 3 Δ–X 3 Δ transition of TiO have been observed using a dispersed laser induced fluorescence (DLIF) technique. From intensity distributions of the present DLIF and previous emission spectra, the dependence of the electronic transition moment, R e( r), for the C 3 Δ–X 3 Δ system was determined as a function of the internuclear distance, r. For the determination of the absolute R e( r) function, the reported radiative lifetimes of v ′=0 and 1 in the C 3 Δ state were included in the analysis; R e( r) was determined as R e( r)=1.3039(49)[1+22.1(28)( r− r 0) 2−150(25)( r− r 0) 3] ( r 0=1.65869 and 1.62093 Å ⩽ r⩽1.80255 Å). The obtained values of R e( r) were simultaneously analyzed with those for the B 3 Π–X 3 Δ system, the hyperfine coupling constants for the X 3 Δ state, and the spin–orbit constants for the X 3 Δ , B 3 Π , and C 3 Δ states to assess the ionic and orbital characters.

Hydrogen bond-induced vibronic mode mixing in benzoic acid dimer: a laser-induced fluorescence study.

Laser-induced dispersed fluorescence spectra of benzoic acid dimer in the cold environment of supersonic jet expansion have been reinvestigated with improved spectral resolution of measurements. The spectra are analyzed with the aid of the normal mode vibrations of the dimer calculated by the ab initio quantum chemistry method at the DFT/B3LYP/6-311+G(*) (*) level of theory. The analysis reveals that the low-frequency intermolecular hydrogen bond modes are mixed extensively with the carboxyl as well as aromatic ring vibrations upon electronic excitation. The mode mixing is manifested as the complete loss of mirror symmetry relation between the fluorescence excitation and dispersed fluorescence spectra of the S(1) origin, and appearance of large number of cross-sequence transitions when the DF spectra are measured by exciting the low-energy vibrations near the S(1) origin. The cross-sequence bands are found in all the cases to be the combinations of two nontotally symmetric fundamentals consisting of one of the intermolecular hydrogen bond modes and the other from the aromatic ring and carboxyl group vibrations. The implications of this mode mixing on the excited state dynamics of the dimer are discussed.