A time‐dependent calculation of the alignment and orientation of the CN fragment of the photodissociation of ICN

Optothermal properties of plasmonic inorganic nanoparticles for photoacoustic applications

Plasmonic systems are becoming a favourable alternative to dye molecules in the generation of photoacoustic signals for spectroscopy and imaging. In particular, inorganic nanoparticles are appealing because of their versatility. In fact, as the shape, size and chemical composition of nanoparticles are directly correlated with their plasmonic properties, the excitation wavelength can be tuned to their plasmon resonance by adjusting such traits. This feature enables an extensive spectral range to be covered. In addition, surface chemical modifications can be performed to provide the nanoparticles with designed functionalities, e.g., selective affinity for specific macromolecules. The efficiency of the conversion of absorbed photon energy into heat, which is the physical basis of the photoacoustic signal, can be accurately determined by photoacoustic methods. This review contrasts studies that evaluate photoconversion in various kinds of nanomaterials by different methods, with the objective of facilitating the researchers' choice of suitable plasmonic nanoparticles for photoacoustic applications.

Importance of the Parallel Component of the Transition Moments to the 1Π1 (5A') and 3Π1 (3A') Excited States of ICN in the Ã-Band Photodissociation

ICN is one of the few simple triatomic molecules whose photodissociation mechanisms have been thoroughly investigated. Since it has a linear structure in the electronic ground state, the dissociation follows a photoexcitation at a linear or slightly bent structure. It is generally believed that the Ã-band consists of the dominant excitation to ³Π₀₊ (4A') with the transition dipole moment (TDM) parallel to the molecular axis ( z), a slightly weaker transition to ¹Π₁ (5A', 4A″), and a much weaker transition to ³Π₁ (3A', 2A″), both of the latter two having perpendicular TDMs. In the present work, we have theoretically studied the geometry dependence of these TDMs and found a pronounced θ (bending angle) dependence in the parallel ( z) component of the TDMs to ¹Π₁ (5A') and ³Π₁ (3A'), both of which should be zero at a linear geometry by symmetry and thus have been previously ignored. We estimated that the z component TDM to ¹Π₁ (5A') has a contribution of 15-20% to the total absorption cross-section at 249 nm at room temperature. Interestingly, the TDM to ³Π₀₊ (4A') does not exhibit such θ dependency and thus has only the z component. We compare the TDMs of ICN and CH₃I molecules having similar excited states. The fact that all the TDMs to 3A', 4A', and 5A' have nonnegligible z components implies the importance of the coherent excitation contributions to various observables of CN fragment, such as the anisotropy parameter, the orientation parameter, and the rotational level distribution as well as the rotational fine structure level distribution.

Potential energy surfaces and nonadiabatic transitions in the asymptotic regions of ICN photodissociation to study the interference effects in the F1 and F2 spin-rotation levels of the CN products

One of the most spectacular yet unsolved problems for the ICN A ~ -band photodissociation is the non-statistical spin-rotation F₁ = N + 1/2 and F₂ = N - 1/2 populations for each rotation level N of the CN fragment. The F₁ /F₂ population difference function f(N) exhibits strong N and λ dependences with an oscillatory behavior. Such details were found to critically depend on the number of open-channel product states, namely, whether both I (² P_3/2 ) and I (² P_1/2 ) are energetically available or not as the dissociation partner. First, in the asymptotic region, the exchange and dipole-quadrupole inter-fragment interactions were studied in detail. Then, as the diabatic basis, we took the appropriate symmetry adapted products of the electronic and rotational wavefunctions for the F₁ and F₂ levels at the dissociation limits. We found that the adiabatic Hamiltonian exhibits Rosen-Zener-Demkov type nonadiabatic transitions reflecting the switch between the exchange interaction and the small but finite spin-rotation interaction within CN at the asymptotic region. This non-crossing type nonadiabatic transition occurs with the probability 1/2, that is, at the diabatic limit through a sudden switch of the quantization axis for CN spin S from the dissociation axis to the CN rotation axis N. We have derived semiclassical formulae for f(N) and the orientation parameters with a two-state model including the 3A' and 4A' electronic states, and with a four-state model including the 3A' through 6A' electronic states. These two kinds of interfering models explain general features of the F₁ and F₂ level populations observed by Zare's group and Hall's group, respectively. © 2018 Wiley Periodicals, Inc.

Coherent and incoherent orientation and alignment of ICN photoproducts

We report extended measurements of the rotational polarization and correlated angular distribution of CN photofragments from ICN photodissociation, with a particular emphasis on the creation and detection of molecular orientation with circularly-polarized light. Doppler profiles of the nascent photoproducts are measured by Frequency-Modulated (FM) transient absorption, and the resulting high signal-to-noise data are valuable for verifying the form of the angular correlations between the recoil velocity, the photofragment rotational angular momentum, and the space-fixed frame defined by the dissociation polarization. A space-fixed bipolar moment notation can be used for an unambiguous characterization of the maximal set of polarization properties that can be created with one-photon excitation and detected with one-photon Doppler-resolved absorption spectroscopy. Relating the observed polarization moments to the various coherent and incoherent, adiabatic and non-adiabatic mechanisms, that have been derived and verified extensively in the case of diatomic photodissociation to polarized atomic fragments, is not unambiguous in the case of diatomic fragments from triatomic precursors. Constraints among various polarization moments confirmed in the case of diatomic dissociation are not confirmed in this triatomic case, where the perpendicular transitions to non-degenerate A' and A'' components of a linear Omega = 1 state are qualitatively different from excitation to degenerate Omega = +/-1 states in a diatomic molecule.