Yehuda Haas

Twin states and conical intersections in linear polyenes

Abstract We suggest for linear conjugated polyenes a twin state model which represents the ground state (S 0 ) and first excited state (S 1 ) as a superposition of mainly two mesomeric structures, the fully spin-paired one and a diradical. This model rationalizes why the bond-length alternation, which is pronounced in S 0 , more or less disappears in S 1 and why the bond-alternation vibration (highest frequency CC stretch) is raised in S 1 and depressed in S 0 . The similarity to the Peierls effect and Kohn anomaly in one-dimensional metals is emphasized. Moreover, the conical intersection between S 2 and S 1 is qualitatively predicted, and invoking in addition, some spectroscopic and other observations and the phase-change rule, that between S 1 and S 0 can also be predicted. Compared with the consideration of densities of states and matrix elements, these intersections more satisfactorily explain the S 2 /S 1 and S 1 /S 0 internal conversions and their dependence on chain length, substituents, solvent and temperature and is furthermore consistent with photochemistry. This also includes an exponential gap rule for the internal-conversion rates, which is derived from a proposed dependence of the energy at the intersection on the S 1 –S 0 energy gap.

The quenching mechanism of electronically excited Rydberg states of nitric oxide

Two photon excited fluorescence (TPEF) is used to measure the quenching cross sections of some NO Rydberg states by a number of molecules. The cross sections obtained are quite large, particularly for the C 2Π and D 2Σ states, often significantly exceeding the gas kinetic value. It is suggested that a mechanism common to many collision partners involves an ion pair intermediate. Using this model, the quenching efficiency is related to the electron affinity and to the weakest bond energy of the quencher.

Lasers and Chemical Change

The dictionary defines laser as a. device for the amplification of light. Clearly there is more to it since the acronym itself stands for light amplification by stimulated emission of radiation. Moreover there must be some source for the enhanced light energy coming out. In a chemical laser this energy is provided by a chemical reaction. Chemical lasers combine the physical mechanism of lasing with the versatility and scope of chemical kinetics and spectroscopy. They are the present-day analogues of the electrochemical cells and history again repeated. In addition to their many practical applications chemical lasers have also led to considerable progress in our understanding of the fundamentals of chemical reactions and of bulk systems in disequilibrium.

Photochemical α-cleavage of ketones : revisiting acetone

The photochemical [small alpha]-cleavage of acetone is analyzed in view of recent results obtained for the isolated molecule in supersonic jets. The fluorescence decay time of the isolated molecule spans a range of more than six orders of magnitude, from approximately 10(-6) s near the origin of the S(0)-S(1) transition to less than 10(-12) s at about 20 kcal x mol(-1) excess energy. In contrast, the decay time of the excited singlet (S(1), (1)n pi) in the bulk is around 10(-9) s and independent of excitation wavelength. Initial excitation to the (1)npi state is followed by internal conversion (IC) to the ground state and intersystem crossing to the lowest-lying triplet. The rate constants of these processes are comparable to the radiative decay rate constant for excess energy up to 7 kcal x mol(-1) above the origin of the S(0)-S(1) transition. Beyond that energy, the triplet state becomes dissociative and the ISC rate becomes much larger than other processes depleting S(1). The primary reaction on the triplet surface is a barrier-controlled alpha-cleavage to form the triplet radical pair CH(3)(*)+ CH(3)CO(*). Direct reaction from the S(1) is negligible, and the non-quenchable reaction (by triplet quenchers) observed in the bulk gas phase is due to hot triplet molecules that dissociate on the timescale of 10(-12) s or less. The singlet-state decay time measured in the bulk (approximately 1-2 ns) arises from collision-induced processes that populate low-lying levels of S(1). The analysis is aided by detailed state-resolved studies on related molecules (in particular formaldehyde and acetaldehyde) whose photophysics and photochemistry parallel those of acetone.

Ab initio study of styrene and β‐methyl styrene in the ground and in the two lowest excited singlet states

The structure and vibrational frequencies of styrene and trans‐β‐methyl styrene in the lowest three singlet states (S0, S1, and S2) have been calculated using ab initio quantum chemical methods. The frequencies are compared with experimental data obtained in the bulk and in a supersonic jet. The calculation shows that in the ground state the molecules have a broad shallow potential as a function of the torsional angle, are essentially planar, but may be slightly bent. In the S1 and S2 states, the molecules are planar; In S1, the main structural change is in the aromatic ring, that is somewhat expanded. In S2, the C=C vinyl double bond elongates, while the C1—Cα single bond becomes shorter, bringing these two bonds to almost equal length. Correlation diagrams connecting ground state vibrational modes with ones belonging to electronically excited states are given; they show that for many out‐of‐plane modes the vibrational frequencies decrease upon electronic excitation. This is accounted for in terms of the...

Radiationless transitions in solutions: Isotope and proximity effects on Dy3+ by C–H and C–N bonds

The lifetime and quantum yield of fluorescence from the 4F9/2 level of Dy3+ have been measured in perprotonated and perdeuterated dimethylsulfoxide and methylcyanide under various excitation conditions. The results are compared to those obtained in light and heavy water and with other rare earth ions and discussed with reference to the modified energy gap law in the theory of radiationless transitions. The specific energy gap in Dy3+ is such that efficiency of quenching depends on matching the electronic energy gap of the ion by a single vibration of C–H, C–D, or C–N and on the distance of the bond from the ion.

Stability of Polynitrogen Compounds: The Importance of Separating the σ and π Electron Systems

Planar N(x) systems such as cyclo-N(5)(-) and N(5)(+) tend to be more stable than nonplanar systems such as the neutral cyclo-N(6). It is proposed that the key to stabilization is the separation of the sigma and pi electron systems. In both cyclo-N(5)(-) and N(5)(+), a six-pi-electron system is created upon either adding to or removing from the cyclo-N(5) radical one electron. Judicious addition of oxygen atoms to polynitrogen ring compounds such as cyclo-N(4) and cyclo-N(6) can increase their thermodynamic and kinetic stabilities, accompanied by only a small reduction in their efficiency as high energy density materials (HEDMs). The properties of some of these compounds are calculated and compared with the parent all-nitrogen compounds. Coordination of one or more oxygen atoms to the ring leads to effective separation of the sigma and pi electron systems helping to stabilize the systems. Natural bond analysis indicates that the exocyclic NO bonds can assume a single or double bond character, depending on the ring system.

Acetone, a laser-induced fluorescence study with rotational resolution at 320 nm

The forbidden S1 <-- S0 transition of acetone has been investigated by laser-induced fluorescence measurements with a resolution of 270 MHz. The rotational structure demonstrates, that (i) one deals with a-type transitions and (ii) there is a strong coupling between the torsional motion of the two CH3 groups and the tunneling, out-of-plane wagging motion (nu(23)) of acetone. The interpretation of torsion-vibrational combination bands is less conclusive and thus the discussion still has a preliminary character.

Gas phase unimolecular decomposition and chemiluminescence of tetramethyldioxetane initiated by a tea CO2 laser

Abstract Time resolved chemiluminescence spectra of pure tetramethyldioxetane are obtained for the first time using pulsed infrared laser excitation. The only major product, acetone, is shown to be formed by at least two pathways: a unimolecular, collision free, decomposition and a collision induced mechanism. Spectral and temporal behaviour indicate that neither singlet nor triplet acetone are the primary products. Other possibilities are discussed.

Two-photon excitation of nitric oxide to levels near and above the dissociation limit

Abstract The A2σ+ (υ = 0, 1, 2, 3), C2Π (υ = 0, 1) and D2Σ+ (υ = 0) states of NO have been populated by two-photon absorption. Low resolution emission spectra from all these states were recorded. The two-photon absorption technique ensures operation in the optically thin regime even for high NO pressures. We report self-quenching rate constants for all the levels studied