ALBERT

Notes: Effects of external magnetic field (H) on intensity and decay of fluorescence of s-triazine vapor have been examined under collision-free conditions following excitation at the individual rotational lines belonging to the 610 or 620 absorption band of the S0→S1 transition. The fluorescence intensity is reduced by H and the value of the fluorescence lifetime is increased by H, as far as the slowly decaying portion is concerned. The efficiency of the magnetic quenching of fluorescence depends not only on the vibrational level, but also on the rotational level excited and a good correlation in rotational state dependence between the fluorescence lifetime at zero field and the efficiency of the magnetic quenching of fluorescence is found at 620. As J' of the excited level increases, the lifetime of the slow fluorescence increases, while the magnetic quenching becomes more effective. It is shown that both external magnetic field and molecular rotation play a role to increase the level density of the triplet state coupled to S1.

Notes: Excited rotational level dependence of the external magnetic field effects both on intensity and on decay of fluorescence of pyrazine vapor has been carefully examined for the zero-point vibrational level in S1 with a field strength of 0–170 G. The magnetic quenching of the slow fluorescence becomes more effective with increasing rotational quantum number J' of the excited level, and the field strength at which the amount of fluorescence quenching becomes one-half of the total amount of quenching at the saturated fields is roughly proportional to (2J'+1)−1. The magnetic quenching is also found to depend on K' of the excited level. The rotational level dependence of the magnetic quenching of the slow fluorescence is related to a difference in the number of the triplet levels coupled to the optically excited singlet rovibronic level, based on the spin decoupling mechanism of the singlet–triplet mixed level.

Notes: External magnetic field effects on yields and decays of fluorescence of pyrimidine vapor on excitation into various rotational levels belonging to the vibrationless level or the 6a1 level of S1 have been studied in a supersonic jet or in a bulk gas at room temperature with a field strength of 0–150 G. The fast component of fluorescence is not affected by an external magnetic field, whereas the slow fluorescence is quenched by a field except for excitation at the R(0) line belonging to the 0–0 transition. The fluorescence quenching is more effective at the 6a1 level than that at 00, indicating that the level density of the triplet state coupled to the singlet state plays an important role in the magnetic mixing of the triplet spin sublevels, in terms of which the fluorescence quenching by a magnetic field is interpreted. The excited rotational level dependence of the fluorescence quenching by a magnetic field is attributed to K scrambling in the triplet manifold following intersystem crossing.

Notes: External magnetic field effects on intensity and decay of fluorescence of pyrazine-d4 have been examined with excitation at the individual rotational lines of the 0–0 band belonging to the S0→S1 transition. A single exponential decay modulated by the quantum beats or a pseudobiexponential decay of fluorescence observed at zero field with excitation into very low rotational levels changes to a biexponential decay, as the strength of the external magnetic field (H) increases. The intensity of the slow component effectively decreases with increasing H, whereas the intensity of the fast component increases with increasing H, though both intensities reach constant values at high fields, respectively. The field-induced change of the fast component becomes smaller with increasing J', whereas the magnetic quenching of the slow component becomes more efficient with increasing J'. The fluorescence lifetime of the slow component of pyrazine-d4 decreases with increasing H and has a tendency to increase with increasing J' both in the absence and in the presence of H. A field-induced mixing between T1(nπ*) and T2(ππ*) is suggested to play a significant role in magnetic field effects on fluorescence of pyrazine-d4.

Notes: Fluorescence intensity and decay of pyrimidine vapor have been measured as a function of external magnetic field (H) with excitation at the individual rotational lines belonging to the 6a20 and 1220 bands of the S0→S1 transition. On excitation into very low rotational levels of 6a2 or 122, dynamics at zero field is characterized by the small molecule limit, where fluorescence exhibits a nearly single exponential decay superimposed by the quantum beats, but the fluorescence decay at the initial stage of time becomes faster with increasing H and the decay profile becomes biexponential at high fields: A magnetic-field-induced change of dynamics from the small molecule behavior to the intermediate case occurs in the S1 state. A field strength required for the change becomes smaller with increasing the excess vibrational energy above the S1 origin (ΔE) and with increasing the rotational quantum number of the excited level (J'). On excitation into higher rotational levels, on the other hand, dynamics at zero field is characterized by the intermediate case, where fluorescence exhibits a biexponential decay, and only the slow component is efficiently quenched by H. Magnetic quenching of fluorescence is confirmed to become more efficient with increasing ΔE and with increasing J'. The efficiency both of the magnetic-field-induced change of dynamics and of the magnetic quenching of fluorescence is related to the level density of the triplet state coupled to S1 at zero field. A field-induced mixing between the triplet rovibrational levels which are coupled and uncoupled to S1, respectively, seems to play a part in magnetic quenching, besides the field-induced mixing among the spin sublevels belonging to the triplet levels coupled to S1 at zero field. On the basis of the rotational state dependence both of the fluorescence decay at zero field and of the magnetic field effects on intensity and decay profile of fluorescence, the relation between level structure and dynamics is discussed.

Notes: Stark quantum beat spectroscopy is applied to pyrimidine vapor in a supersonic jet with excitation at the R(0) and R(1) rotational lines of the 0–0 band belonging to the S0→S1 transition. The dependence of the amplitude and phase of the Stark quantum beat on the polarization of both incident light and emission as well as on the geometry with respect to excitation and detection is theoretically predicted, and the results are useful for identification of the observed Stark quantum beats. The electric dipole moment in the S1 excited state of pyrimidine vapor is evaluated to be 0.58 D, consistent results being obtained from experiments with the R(0) and R(1) excitations. A marked decrease of the dipole moment in going from S0 to S1, i.e., from 2.334 to 0.58 D, is consistent with the n→π* transition.

Notes: The magnetic quenching of fluorescence in intermediate case molecules is modeled by including two triplet manifolds {||bj〉} and {||cj〉} mutually shifted by the zero-field splitting Egap (though a triplet has three spin sublevels); the {||bj〉} are coupled to a bright singlet state ||s〉 by intramolecular interaction V and the two manifolds are coupled by a magnetic field. For the two manifold Bixon–Jortner model where the level spacings and the couplings to ||s〉 are constant and no spin–vibration interactions exist (the Zeeman interaction connects only the spin sublevels of the same rovibronic level j), there are two sets of field dressed eigenstates, {||bˆj〉} and {||cˆj〉}, of the background Hamiltonian H−V. ||bˆj〉 and ||cˆj〉 are liner combinations of ||bj〉 and ||cj〉. We call the energy structure "eclipsed (E)'' when the two sets of dressed states overlap in energy and call it "staggered (S)'' when every ||bˆ〉 state is just between two adjacent ||cˆ〉 states.The E and S structures alternatively appear with increasing Zeeman energy hZ. As hZ increases, the number of effectively coupled background levels, Neff, increases for the S structure but remains unchanged for the E structure. The S structure is in accord with the experimental result that the quantum yield is reduced to 1/3 at anomalously low fields (hz/Egap(very-much-less-than)1): in the far wing regions of the absorption band the mixing between the manifolds is determined by the ratio hZ/Egap, but near the band center the intermanifold mixing is enhanced by the presence of ||s〉. Using a random matrix approach where H is constructed of the rotation–vibration Hamiltonians HB and HC arising from the manifolds {||bj〉} and {||cj〉}, we show that an S structure can be formed in real molecules by nonzero ΔHBC≡HB−HC−Egap (Egap is the zero-field splitting at the equilibrium nuclear configuration). Indirect spin–vibration interactions lead to ΔHBC≠0; the vibrational ΔHBC caused by spin–spin and vibronic interactions and the rotational ΔHBC caused by spin–rotation and rotation–vibration interactions. The matrix elements of H are written down in terms of the eigenfunctions {||j〉} of the average Hamiltonian (HB+HC)/2. If the vibrational modes are strongly coupled (the energies of levels are given by a Wigner distribution and the coupling strengths are given by a Gaussian distribution), the vibrational 〈j||ΔHBC||j′〉 for wave functions of roughly the same energy are Gaussian random.As the rms of 〈j||ΔHBC||j′〉 approaches the average level spacing (on excitation into higher vibrational levels), the efficiency of magnetic quenching becomes as high as in the S case. Nonzero 〈j||ΔHBC||j′〉 let isoenergetic levels belonging to different manifolds vibrationally overlap: the ΔHBC, together with the magnetic field, causes level repulsion leading to the S structure and opens up isoenergetic paths between the manifolds. The efficient magnetic quenching in pyrazine can be explained by the vibrational ΔHBC, since the S1–T1 separation is as large as 4500 cm−1. If Coriolis couplings cause K scrambling considerably, the rotational ΔHBC mixes {||j〉}. This mechanism explains the rotational dependence of magnetic quenching in s-triazine of which S1–T1 separation is only ∼1000 cm−1. © 1995 American Institute of Physics.