Isotopic Analysis of Lead at Ultratrace Levels Using Thermal Ionization Mass Spectrometry (TIMS) Coupled with the Continuous Heating Method: Optimization of the Data Integration Range and Method.

This study systematically and experimentally evaluates data integration methods for the isotopic analysis of Pb at ultratrace levels using thermal ionization mass spectrometry (TIMS) with a continuous heating method. The evaluation utilized a certified reference material of Pb (SRM 981). The experimental evaluations encompass different data calculation methods (methods I, II, and III) and integration ranges (full, over 1%, 25%, and 75%). Method I, in which isotope ratios were calculated based on summed ion signal intensities compensating for mass fractionation, was consistent with the certified values for 10 and 1 ng standard samples across all integration ranges. For 100 pg samples, full range calculations failed for specific isotope ratios, but reduced ranges (over 1%, 25%, and 75%) yielded values overlapping with certified ones. Method II, in which isotope ratios were calculated by averaging the precalculated isotope ratios, exhibited inferior performance compared to method I. Method III, using weighted averaging to reduce anomalous values, showed results consistent with those of method I but was recommended only for single measurements. An integration range of over 1% or 25% is preferred to exclude anomalies while compensating for mass fractionation. The optimized method was validated by comparing two different instruments used for the isotopic analysis of the reference material. The enhanced accuracy and precision provide valuable insights for researchers working in ultratrace-level Pb isotopic analysis using TIMS.

Experimental Observation of the Autler-Townes Splitting in Polyatomic Molecules.

Autler-Townes (AT) splitting has been experimentally observed in the optical transition between the zero-point levels of S1 and S0 for supersonically cooled 2-methoxythiophenol, 2-fluorothiophenol, and 2-chlorothiophenol. This is the first experimental observation of the light-dressed quantum states of polyatomic molecules (N > 3) in the electronic transition. In the resonance-enhanced ionization process involving the optically coupled states, if Rabi cycling is ensured within the nanosecond laser pulse, AT splitting is clearly observed for the open system for which the excited-state lifetime is shorter than hundreds of picoseconds. Semiclassical optical Bloch equations and a dressed-atom approach based on the three-level atomic model describe the experiment quite well, giving deep insights into the light-matter interaction in polyatomic molecular systems.

Conformer Specific Excited-State Structure of 3-Methylthioanisole.

Trans and cis conformers of 3-methylthioanisole have been spectroscopically investigated to reveal the conformer specific structural changes upon the S1(ππ*)-S0 excitation. The conformational cooling during the supersonic expansion is found to be quite efficient in the Ar carrier gas giving the trans conformational isomer exclusively in the molecular beam, whereas both trans and cis conformers are populated in the jet when the sample is carried in Ne. Using the Stark deflector, trans and cis conformers are unambiguously identified, showing the distinct Stark deflection profiles according to their sufficiently different dipole moments of 1.013 or 1.670 D, respectively. For the trans conformer, the methyl moiety on the meta-position adopting the eclipsed geometry in S0 transforms into the staggered geometry in S1 to activate a series of the CH3 torsional mode. A Hamiltonian with the one-dimensional sinusoidal torsional potential is solved using the free-rotor basis set to explain the experiment, giving the 3-fold torsional barrier of 34 and 304 cm-1 for S0 and S1, respectively. For the cis conformer, on the other hand, the CH3 torsion is little activated in the S1-S0 transition as both S0 and S1 adopt the staggered geometry at the minimum energy points. The doublet of each band of the cis conformer is ascribed to tunneling split due to the very low CH3 torsional barrier of 27 cm-1 in S0. It is found that the cis conformer undergoes a planar to pseudoplanar structural change upon the S1-S0 transition. Theoretical calculation based on the double-well model potential curve could explain the experiment quite well, suggesting that the SCH3 moiety of the cis conformer in S1 becomes out-of-plane with respect to the plane of the phenyl moiety. This implies that excited-state predissociation dynamics of trans and cis conformers of the title molecule might be different.

Photodissociation Dynamics of Ortho-Substituted Thiophenols at 243 nm.

The photoinduced S-H (D) bond fission dynamics of four ortho-substituted thiophenols, 2-fluoro, 2-chloro, 2-bromo, and 2-methoxythiophenol at a pump wavelength of 243 nm, have been investigated by velocity-map imaging and high-level electronic structure calculations. The D atom images of the deuterated ortho-substituted thiophenols show much reduced X̃/à branching ratios of the cofragment radicals over that of bare thiophenol. The angular distributions of the D fragment display negative anisotropies, indicating that transition dipole moments are perpendicular to the fast dissociating S-D bond axis. Initial excitation at 243 nm occurs directly to the 1πσ* state or to the 21ππ* state followed by efficient coupling to the 1πσ* state. The calculated potential energy curves for the 1πσ* or 21ππ* excited states of the ortho-substituted thiophenols along the CCS-D torsion angle (ϕ) display minima at the nonplanar structures, whereas all of the states for bare thiophenol present minima at the planar geometries. This different topology of the ortho-substituted thiophenols in the excited states induces the wide spread of the reactive flux along the ϕ coordinate on the repulsive surface as it should experience significant torque with respect to ϕ during the fragmentation. This encourages the dissociating molecules to follow the adiabatic path at the conical intersection between the ground and the 1πσ* states at extended S-D bond lengths, giving rise to decreased X̃/à branching ratios, demonstrating that the excited-state molecular structure dictates the nonadiabatic transition probability.

Experimental observation of nonadiabatic bifurcation dynamics at resonances in the continuum.

The surface crossing of bound and unbound electronic states in multidimensional space often gives rise to resonances in the continuum. This situation happens in the πσ*-mediated photodissociation reaction of 2-fluorothioanisole; optically-bright bound S1 (ππ*) vibrational states of 2-fluorothioanisole are strongly coupled to the optically-dark S2 (πσ*) state, which is repulsive along the S-CH3 elongation coordinate. It is revealed here that the reactive flux prepared at such resonances in the continuum bifurcates into two distinct reaction pathways with totally different dynamics in terms of energy disposal and nonadiabatic transition probability. This indicates that the reactive flux in the Franck-Condon region may either undergo nonadiabatic transition funneling through the conical intersection from the upper adiabat, or follow a low-lying adiabatic path, along which multiple dynamic saddle points may be located. Since 2-fluorothioanisole adopts a nonplanar geometry in the S1 minimum energy, the quasi-degenerate S1/S2 crossing seam in the nonplanar geometry, which lies well below the planar S1/S2 conical intersection, is likely responsible for the efficient vibronic coupling, especially in the low S1 internal energy region. As the excitation energy increases, bound-to-continuum coupling is facilitated with the aid of intramolecular vibrational redistribution, along many degrees of freedom spanning the large structural volume. This leads to the rapid domination of the continuum character of the reactive flux. This work reports direct and robust experimental observations of the nonadiabatic bifurcation dynamics of the reactive flux occurring at resonances in the continuum of polyatomic molecules.

Vibronic structure and predissociation dynamics of 2-methoxythiophenol (S1): The effect of intramolecular hydrogen bonding on nonadiabatic dynamics.

Vibronic spectroscopy and the S-H bond predissociation dynamics of 2-methoxythiophenol (2-MTP) in the S1 (ππ*) state have been investigated for the first time. Resonant two-photon ionization and slow-electron velocity map imaging (SEVI) spectroscopies have revealed that the S1-S0 transition of 2-MTP is accompanied with the planar to the pseudoplanar structural change along the out-of-plane ring distortion and the tilt of the methoxy moiety. The S1 vibronic bands up to their internal energy of ∼1000 cm-1 are assigned from the SEVI spectra taken via various S1 vibronic intermediate states with the aid of ab initio calculations. Intriguingly, Fermi resonances have been identified for some vibronic bands. The S-H bond breakage of 2-MTP occurs via tunneling through an adiabatic barrier under the S1/S2 conical intersection seam, and it is followed by the bifurcation into either the adiabatic or nonadiabatic channel at the S0/S2 conical intersection where the diabatic S2 state (πσ*) is unbound with respect to the S-H bond elongation coordinate, giving the excited (Ã) or ground (X̃) state of the 2-methoxythiophenoxy radical, respectively. Surprisingly, the nonadiabatic transition probability at the S0/S2 conical intersection, estimated from the velocity map ion images of the nascent D fragment from 2-MTP-d1 (2-CH3O-C6H4SD) at the S1 zero-point energy level, is found to be exceptionally high to give the X̃/à product branching ratio of 2.03 ± 0.20, which is much higher than the value of ∼0.8 estimated for the bare thiophenol at the S1 origin. It even increases to 2.33 ± 0.17 at the ν45 2 mode (101 cm-1) before it rapidly decays to 0.69 ± 0.05 at the S1 internal energy of about 2200 cm-1. This suggests that the strong intramolecular hydrogen bonding of S⋯D⋯OCH3 in 2-MTP at least in the low S1 internal energy region should play a significant role in localizing the reactive flux onto the conical intersection seam. The minimum energy pathway calculations (second-order coupled-cluster resolution of the identity or time-dependent-density functional theory) of the adiabatic S1 state suggest that the intimate dynamic interplay between the S-H bond cleavage and intramolecular hydrogen bonding could be crucial in the nonadiabatic surface hopping dynamics taking place at the conical intersection.

Spatial Isolation of Conformational Isomers of Hydroquinone and Its Water Cluster Using the Stark Deflector.

Conformational isomers of hydroquinone and their 1:1 clusters with water have been spatially separated using a Stark deflector in a supersonic jet. trans-Hydroquinone (HyQ) conformer with zero dipole moment is little influenced by inhomogeneous electric fields, whereas cis conformer with nonzero dipole moment (2.38 D) is significantly deflected from the molecular beam axis into the direction along which the strong field gradient is applied. Resonant two photon ionization carried out by shifting the laser position perpendicular to the molecular beam axis after the Stark deflector then gives an exclusive S1-S0 excitation spectrum of the cis conformer only, making possible immaculate conformer-specific spectroscopy and dynamics. As the spatial separation is apparently proportional to the effective dipole moment strength, conformational assignment could be absolute in the Stark deflector, which contrasts with the hole-burning spectroscopic technique where identification of a conformational isomer is intrinsically not unambiguous. trans- and cis-HyQ-H2O clusters have also been spatially separated according to their distinct effective dipole moment strengths to give absolute spectroscopic identification of each cluster isomer, nailing down the otherwise disputable conformational assignment. This is the first report for the spatial separation of conformational cluster isomers.