Excited Pyrazine Research Articles

UV Photolysis of Pyrazine and the Production of Hydrogen Isocyanide.

Photolysis of the diazine heterocycle, pyrazine, following irradiation at 308, 248, and 193 nm was examined using nanosecond time-resolved Fourier transform infrared emission spectroscopy. The resulting time-resolved IR emission spectra reveal that for 308 and 248 nm vibrationally highly excited pyrazine is produced, but no photolysis products were detected. However, at 193 nm excitation, the measured IR emission spectra consist solely of resonances originating from rovibrationally excited photofragments, including acetylene (HCCH), hydrogen cyanide (HCN), and hydrogen isocyanide (HNC), indicating that photofragmentation proceeds from vibrationally highly excited pyrazine on the ground electronic state. Spectral fit analysis of the time-resolved HCN and HNC IR emission band shapes and intensities allowed an estimate of the nascent product population distributions, from which a lower bound estimate of the HNC/HCN branching ratio was deduced as Φ ≥ 0.07. Additionally, ab initio calculations were performed in order to examine the propensity of photoinduced reactions on the ground- and lowest-energy excited-state surfaces. The calculations provide a basis for understanding the wavelength dependence of the UV photolysis of pyrazine, the photolytic production of HNC, and also explain previous experimental observations in the literature.

Performance of a high-resolution mid-IR optical-parametric-oscillator transient absorption spectrometer.

We report on a mid-IR optical parametric oscillator (OPO)-based high resolution transient absorption spectrometer for state-resolved collisional energy transfer. Transient Doppler-broadened line profiles at λ = 3.3 μm are reported for HCl R7 transitions following gas-phase collisions with vibrationally excited pyrazine. The instrument noise, analyzed as a function of IR wavelength across the absorption line, is as much as 10 times smaller than in diode laser-based measurements. The reduced noise is attributed to larger intensity IR light that has greater intensity stability, which in turn leads to reduced detector noise and better frequency locking for the OPO.

Full State-Resolved Energy Gain Profiles of CO2 from Collisions with Highly Vibrationally Excited Molecules. II. Energy-Dependent Pyrazine (E = 32 700 and 37 900 cm–1) Relaxation

The full state-resolved distribution of scattered CO2 (00(0)0) molecules from collisions with highly vibrationally excited pyrazine (E = 32,700 cm(-1)) is reported and compared to previous studies on pyrazine (E = 37,900 cm(-1)) to investigate how internal energy content impacts the dynamics for collisional quenching of high energy molecules [J. Phys. Chem. A 2010, 113, 1569]. Nascent rotational and translational energy profiles for scattered CO2 (00(0)0) molecules with J = 2-72 were measured using high-resolution transient infrared absorption and combined with earlier results for the J = 56-78 states [J. Chem. Phys. 1999, 111, 7373]. The product translational energy for individual J-states increases by 50% for a 16% increase in donor vibrational energy. The nascent rotational distribution for scattered CO2 is biexponential, comprising 77% nearly elastic collisions and 23% inelastic collisions. The spread of the rotational distribution is sensitive to donor energy, but the branching ratio for elastic and inelastic collisions is the same for both donor energies. The measured collision rates are close to the Lennard-Jones values and are only weakly dependent on changes in donor energy. The nascent energy gain distribution function P(ΔE) depends strongly on the energy, and this energy dependence is stronger than the linear dependence seen in multicollision energy transfer studies for pyrazine(E) + CO2 collisions.

Dynamics of Weak and Strong Collisions: Highly Vibrationally Excited Pyrazine (E = 37900 cm−1) with DCl

The outcome of energy transfer due to single collisions between highly vibrationally excited pyrazine (E = 37,900 cm(-1)) and DCl is measured for the appearance of individual low-J rotational states of DCl using high-resolution transient IR absorption. Appearance profiles from double-Gaussian transient lineshapes were measured for a number of DCl states with J = 2-12. These data give information on the recoil velocity distributions, appearance rates, and populations of individual states of the scattered molecules. These data complement previous studies on high-J state DCl scattering (Li, Z.; Korobkova, E.; Werner, K.; Shum, L.; Mullin, A. S. J. Chem. Phys. 2005, 123, 174306), and together, they provide a full description of the V-RT collisions with DCl that quench pyrazine(E). Scattered DCl (v = 0) molecules with J = 2-21 are rotationally hot with T(rot) = 880 +/- 100 K. Center-of-mass translational energy distributions are T(rel) approximately 700 K for J < 15. Broader velocity distributions are observed for the J = 15-20 states. The rate constant for V-RT energy transfer is 4.6 x 10(-10) cm(3) molecule(-1) s(-1). This value is a lower limit to the overall rate constant for energy transfer and corresponds to approximately 85% of the Lennard-Jones collision rate. We estimate scattering into the DCl (v = 1) state occurs in approximately 1% of collisions. The energy transfer probability distribution P(DeltaE) is presented and yields DeltaE = 888 cm(-1).

Collisions of Highly Vibrationally Excited Pyrazine (Evib = 37 900 cm-1) with HOD: State-Resolved Probing of Strong and Weak Collisions

This work presents state-resolved measurements of weak and strong collisions between HOD and highly vibrationally excited pyrazine (Evib = 37,900 cm(-1)). Transient IR absorption line profiles of HOD(000) rotational states that are populated in single collisions with pyrazine (Evib) are fit using double-Gaussian functions to extract Doppler-broadened line widths and energy transfer rates for appearance and depletion populations. We recently reported the use of this new approach to determine collision rates for energy transfer (Havey, D. K.; Liu, Q.; Li, Z. M.; Elioff, M.; Fang, M.; Neudel, J.; Mullin, A. S. J. Phys. Chem. A 2007, 111, 2458-2460). Here we present a full description of transient measurements of weak collisions, along with rate constants and the full energy transfer distribution function for the vibrational-to-rotation/translation (V-RT) pathway. The low- and high-J populations of scattered HOD(000) are characterized by a single rotational distribution with Trot = 430 +/- 50 K. The average translational energy of the scattered HOD(000) increases modestly with rotational energy suggesting an impulsive energy transfer mechanism. The energy gain distribution P(Delta E) for HOD(000) shows that approximately 99% of collisions have DeltaE values less than 3000 cm(-1). These data yield a lower limit to the collision rate that is approximately 70% larger than the calculated Lennard-Jones collision rate. These findings show that water is an efficient quencher due to a large collisional energy transfer cross section and not as a result of a large supercollision tail.

Direct Determination of Collision Rates beyond the Lennard-Jones Model through State-Resolved Measurements of Strong and Weak Collisions

We describe a new approach for measuring absolute rates for molecular collisions including contributions of both strong and weak collisions. Elastic and inelastic collisions are monitored using high-resolution transient IR spectroscopy by measuring increases in the velocity distributions of individual rotational states of scattered molecules. Weak collisional energy transfer is detected by measuring velocity increases for the low-energy rotational states. This technique is illustrated for the collisional relaxation of highly vibrationally excited pyrazine (108 kcal/mol) with HOD. The observed collision rate is nearly twice the Lennard-Jones collision rate.

State-resolved collisional quenching of vibrationally excited pyrazine (Evib=37900cm−1) by D35Cl(v=)

Supercollision relaxation of highly vibrationally excited pyrazine (E(vib) = 37,900 cm(-1)) with D35Cl is investigated using high-resolution transient IR diode laser absorption spectroscopy at 4.4 microm. Highly excited pyrazine is prepared by pulsed UV excitation at 266 nm, followed by rapid radiationless decay to the ground electronic state. The rotational energy distribution of the scattered DCl (v = 0,J) molecules with J = 15-21 is characterized by T(rot) = 755+/-90 K. The relative translational energy increases as a function of rotational quantum number for DCl with T(rel) = 710+/-190 K for J = 15 and T(rel) = 1270+/-240 K for J = 21. The average change in recoil velocity correlates with the change in rotational angular momentum quantum number and highlights the role of angular momentum in energy gain partitioning. The integrated energy-transfer rate for appearance of DCl (v = 0,J = 15-21) is k(2)(int) = 7.1x10(-11) cm3 molecule(-1) s(-1), approximately one-eighth the Lennard-Jones collision rate. The results are compared to earlier energy gain measurements of CO2 and H2O.

Trajectory Study of Supercollision Relaxation in Highly Vibrationally Excited Pyrazine and CO2

Classical trajectory calculations were performed to simulate state-resolved energy transfer experiments of highly vibrationally excited pyrazine (E(vib) = 37,900 cm(-1)) and CO(2), which were conducted using a high-resolution transient infrared absorption spectrometer. The goal here is to use classical trajectories to simulate the supercollision energy transfer pathway wherein large amounts of energy are transferred in single collisions in order to compare with experimental results. In the trajectory calculations, Newton's laws of motion are used for the molecular motion, isolated molecules are treated as collections of harmonic oscillators, and intermolecular potentials are formed by pairwise Lennard-Jones potentials. The calculations qualitatively reproduce the observed energy partitioning in the scattered CO(2) molecules and show that the relative partitioning between bath rotation and translation is dependent on the moment of inertia of the bath molecule. The simulations show that the low-frequency modes of the vibrationally excited pyrazine contribute most to the strong collisions. The majority of collisions lead to small DeltaE values and primarily involve single encounters between the energy donor and acceptor. The large DeltaE exchanges result from both single impulsive encounters and chattering collisions that involve multiple encounters.

Long Range Collisional Energy Transfer from Highly Vibrationally Excited Pyrazine to CO Bath Molecules: Excitation of the v = 1 CO Vibrational Level

Collisional energy transfer from highly vibrationally excited pyrazine to CO bath molecules has been studied by use of an infrared diode laser to probe the vibrational, rotational, and translation degrees of freedom of scattered CO molecules. Here the excitation of the CO (v = 1) bath molecule vibrational state is investigated after a collision with highly excited, vibrationally “hot” pyrazine. Only a small amount of rotational and translational excitation accompanies the production of CO (v = 1), implicating a “soft” collision, long-range force energy transfer mechanism. The amount of CO scattered into the v = 1 state at 2170 cm-1 is small when compared to the amount of CO2 scattered into the analogous 0001 level of CO2 at 2349 cm-1 by collisions with pyrazine molecules having 40 640 cm-1 of internal vibrational energy.

Collisional energy transfer of highly vibrationally excited toluene and pyrazine: Transition probabilities and relaxation pathways from KCSI experiments and trajectory calculations

New experimental results for the collisional energy transfer of highly vibrationally excited toluene and pyrazine employing the method of “kinetically controlled selective ionization (KCSI)” are presented. By means of a master equation approach we determine complete and detailed collisional transition probabilities P(E′,E) for energies up to 50000 cm−1. The same monoexponential representation P(E′,E)∝exp[ − ((E − E′)/α1(E))Y] (for E′⩽E) with a parametric exponent Y in the argument and linearly energy dependent α1(E) = C0 + C1E successfully used in our earlier investigation [T. Lenzer, K. Luther, K. Reihs and A. C. Symonds, J. Chem. Phys., 2000, 112, 4090] can reproduce the toluene and pyrazine results for the whole range of bath gases studied. The parameters Y, C0 and C1 of P(E′,E) show a smooth increase with the size of the collider. An approximately linear energy dependence of the first moment of energy transfer 〈ΔE〉 is observed for all bath gases. Literature data from infrared fluorescence (IRF) experiments in general show significantly smaller − 〈ΔE〉 values outside the uncertainty limits of the KCSI results. It is shown that this can mainly be traced back to the critical dependence of the IRF data on small uncertainties in the calibration curve. Some of the trends with respect to the energy transfer efficiencies of different colliders observed in the KCSI experiments are easily rationalized on the basis of accompanying trajectory calculations on the deactivation of highly vibrationally excited pyrazine by n-propane and CO2. The negligible influence of the V–V relaxation channel in the pyrazine + CO2 system observed in earlier IR diode laser studies is confirmed.

Vibrational Energy Gain in the ν2 Bending Mode of Water via Collisions with Hot Pyrazine (Evib = 37900 cm-1): Insights into the Dynamics of Energy Flow

Energy gain into specific rotational levels of vibrationally excited H2O(010) resulting from collisions with highly vibrationally excited pyrazine in a low-pressure, 298 K environment was investigated using high-resolution transient infrared absorption spectroscopy of water at λ ≈ 2.7 μm. Vibrationally excited pyrazine with 37900 cm-1 internal energy was generated by 266 nm optical excitation to an electronically excited singlet state of pyrazine, followed by rapid radiationless decay to the ground electronic state. Collisions between highly excited pyrazine and water that result in excitation of the ν2 bending vibrational mode (ν2 = 1594 cm-1) in water were studied by monitoring the time-resolved appearance of individual rotational states of H2O(010). Transient absorption signals were obtained for a number of rotational states with Erot ≤ 811 cm-1. The nascent distribution of rotational states for the scattered, vibrationally excited water molecules, H2O(010), is well characterized by a Boltzmann rotatio...

Competition between photochemistry and energy transfer in ultraviolet-excited diazabenzenes. II. Identifying the dominant energy donor for “supercollisions”

CO 2 bath molecules scattered into J=72 of the 0000 vibrational state at short times after 248 or 266 nm UV excitation of pyrazine are probed using high resolution time resolved IR diode laser spectroscopy as a function of UV laser fluence from ∼3 to 80 mJ/cm2. The implications of pyrazine photodissociation for the interpretation of these collisional energy transfer experiments are considered. Specifically, the possibility that translationally hot HCN resulting from pyrazine dissociation may be the source of excitation for collisions that impart a large amount of rotational and translational energy to CO2 molecules is examined. Transient absorption measurements probing rotationally and translationally excited CO2 molecules produced following excitation of pyrazine are analyzed within the context of a kinetic scheme incorporating pyrazine photodissociation, as well as excitation of CO2 by both translationally hot HCN and vibrationally excited pyrazine. This analysis indicates that vibrationally hot pyrazine, which has sufficient energy to dissociate, is the source of excitation in collisions imparting large amounts of rotational and translational energy to CO2.

Electron configuration changes in excited pyrazine molecules analyzed by femtosecond time-resolved photoelectron spectroscopy

Using the pump–probe technique with 130 fs laser pulses near 200 nm and near 266 nm the internal conversion of the pyrazine molecule excited to the S2 state has been studied. The lifetime of the S2 state due to internal conversion to the lower electronic states is τIC(2)=(20±10) fs while the lifetime of the secondarily populated S1 state is τIC(1)=(22±1) ps. The results of femtosecond time-resolved electron spectroscopy directly demonstrate the variation of the electron configuration during the internal conversion: The electron spectrum changes significantly on the fs time scale for pyrazine ions produced by ionization via the S2 state with ππ* character and by ionization of S1 state molecules with nπ* configuration after the internal conversion, respectively. The results obtained confirm theoretical estimations of Domcke and co-workers [J. Chem. Phys. 95, 7806 (1991); J. Phys. Chem. 97, 12466 (1993)] who describe the internal conversion in the pyrazine molecule on the basis of a conical intersection of the corresponding potential energy surfaces.

Unraveling the energy dependence in large ΔE (V→RT) energy transfer: Separation of ΔE and probability in the collisional relaxation of highly vibrationally excited pyrazine (Evib=36 000 to 41 000 cm−1) by CO2

The energy dependence of collisional relaxation of highly vibrationally excited pyrazine through collisions with CO2 has been investigated for pyrazine vibrational energies Evib=36 000–41 000 cm−1. Highly excited pyrazine was prepared by absorption of tunable pulsed UV laser light at four wavelengths between 251 and 271 nm. Data for pyrazine excitation at 246 and 266 nm have been published previously and are included with the data presented here. We have used high resolution transient absorption spectroscopy to measure nascent distributions of rotational and translational energy gain in CO2 molecules in their ground vibrationless (0000) state that are populated through collisions with excited pyrazine molecules. Our results reveal that substantial amounts of rotational and translational energy are imparted to CO2(0000) molecules for all pyrazine energies studied and that the magnitude of this energy changes very little for a 13% change in the pyrazine vibrational energy. In contrast, state-resolved rates for appearance of individual CO2(0000) rotational states show a strong dependence on the precise value of the donor energy and increase by nearly an order-of-magnitude for a 5000 cm−1 increase in pyrazine energy. The high energy part of P(E,E′) is obtained from our energy gain measurements for each pyrazine excitation energy and these distributions are compared with experimental measurements of 〈ΔE〉 and its energy dependence.

State-resolved collisional quenching of highly vibrationally excited pyridine by water: The role of strong electrostatic attraction in V→RT energy transfer

We report experiments that investigate the influence of long-range attractive forces on collisional energy loss from highly vibrationally excited molecules. State-resolved studies of energy transfer from highly vibrationally excited pyridine (μ=2.2 D) to water (μ=1.8 D) in a low-pressure environment at 298 K have been performed using high-resolution transient absorption spectroscopy of water at λ≈2.7 μm. Pyridine in its ground electronic state with 37 900 cm−1 of vibrational energy was prepared by absorption of pulsed ultraviolet light (λ=266 nm) to the S1 state, followed by rapid internal conversion to the S0 state. Collisions between vibrationally excited pyridine and water that result in rotational and translational excitation of the ground vibrationless state of H2O (000) were investigated by monitoring the populations of individual rotational states of H2O (000) at short times following pyridine excitation. The infrared probe of water was the highly allowed asymmetric stretching (000→001) transition. The nascent distribution of rotationally excited H2O (000) states is well described by a thermal distribution with a rotational temperature of Trot=770±80 K. Doppler-broadened transient linewidth measurements yield the velocity distributions of the recoiling H2O (000) molecules that correspond to center-of-mass translational temperatures of Ttrans∼515 K for all water rotational states investigated. Additionally, rate constants for energy gain in individual water states were determined, yielding an integrated rate constant of k2int=1.1×10−11 cm3 mol−1 s−1 for the appearance of H2O (000) with Erot=1000–2000 cm−1. These results are compared with previous relaxation studies of excited pyrazine (μ=0 D) with water and of excited pyridine with CO2 (μ=0 D), and the influence of electrostatic attraction on the relaxation dynamics is discussed.

Observation of an energy threshold for large ΔE collisional relaxation of highly vibrationally excited pyrazine (Evib=31 000–41 000 cm−1) by CO2

Energy dependent studies of the collisional relaxation of highly vibrationally excited pyrazine through collisions with CO2 were performed for initial pyrazine energies Evib=31 000–35 000 cm−1. These studies are presented along with earlier results for pyrazine with Evib=36 000–41 000 cm−1. High-resolution transient IR laser absorption of individual CO2 (0000) rotational states (J=56–80) was used to investigate the magnitude and partitioning of energy gain into CO2 rotation and translation, which comprises the high energy tail of the energy transfer distribution function. Highly vibrationally excited pyrazine was prepared by absorption of pulsed UV light at seven wavelengths in the range λ=281–324 nm, followed by radiationless decay to pyrazine’s ground electronic state. Nascent CO2 (0000) rotational populations were measured for each UV excitation wavelength and distributions of nascent recoil velocities for individual rotational states of CO2 (0000) were obtained from Doppler-broadened transient linewidth measurements. Measurements of energy transfer rate constants at each UV wavelength yield energy-dependent probabilities for collisions involving large ΔE values. These results reveal that the magnitude of large ΔE collisional energy gain in CO2 (0000) is fairly insensitive to the amount of vibrational energy in pyrazine for Evib=31 000–35 000 cm−1. A comparison with earlier studies on pyrazine with Evib=36 000–41 000 cm−1 indicates that the V→RT energy transfer increases both in magnitude and probability for Evib&amp;gt;36 000 cm−1. Implications of incomplete intramolecular vibrational relaxation, electronic state coupling, and isomerization barriers are discussed in light of these results.

State-Resolved Studies of Collisional Quenching of Highly Vibrationally Excited Pyrazine by Water: The Case of the Missing V → RT Supercollision Channel

The quenching of highly vibrationally excited pyrazine through collisions with H2O at 300 K in a low-pressure environment was investigated using high-resolution transient absorption spectroscopy of water at λ ≈ 2.7 μm. Highly vibrationally excited pyrazine with Evib = 37 900 cm-1 was prepared by absorption of 266 nm light to the electronically excited S2 state, followed by rapid radiationless decay to the ground electronic state. Collisions between highly excited pyrazine and water that result in rotational and translational excitation of the vibrationless ground state of H2O (000) were investigated by measuring the state-resolved appearance of individual rotational states of H2O (000). Transient absorption measurements have been made on numerous rotational states to determine the nascent distribution of rotational energy gain in water. Doppler-broadened transient absorption line shapes were collected for a number of rotational levels in the (000) state in order to measure velocity distributions of the scattered water molecules. The nascent distribution of water rotational states with Erot > 1000 cm-1 is well described by Trot = 920 K, and the velocity distributions correspond to Ttrans ≈ 560 K, independent of the rotational state. Rate constants for energy gain into individual quantum states of H2O (000) from collisions with hot pyrazine provide a measure of the high-energy part of the energy-transfer probability distribution function. The quenching of hot pyrazine through collisions with water displays a significant reduction in the bath translational energy gain when compared to earlier studies on the quenching of hot pyrazine (Evib = 37 900 cm-1) by CO2 {Wall, M. C.; Mullin, A. S. J. Chem. Phys. 1998, 108, 9658}. A comparison of the two systems provides insights into the molecular properties that influence the relaxation of highly vibrationally excited molecules.

Independent Determination of Supercollision Energy Loss Magnitudes and Rates in Highly Vibrationally Excited Pyrazine with Evib = 36000−41000 cm-1

We report results of the first state-resolved study of large ΔE energy transfer resulting from collisions of highly excited pyrazine with initial vibrational energies of 36000−41000 cm-1. We have used transient infrared absorption spectroscopy to monitor the rotational and translational energy gain in a CO2 bath at short times following collisions with highly excited pyrazine. We have measured nascent distributions of high-energy rotational states, J = 56−84, for CO2 in its ground vibrationless (0000) state, the translational energy gain associated with these excited rotational states, and state-resolved rate constants for excitation of the high-energy bath states. Our results show that for the CO2 rotational states investigated, the nascent rotational distributions and translational energies change very little for a 13% change in pyrazine energy. In contrast, the rate constants for these events are strongly influenced by the donor internal energy content and increase by an order of magnitude for a 5000 c...

“Supercollision” energy dependence: State-resolved energy transfer in collisions between highly vibrationally excited pyrazine (Evib=37 900 cm−1 and 40 900 cm−1) and CO2

The quenching of highly vibrationally excited pyrazine through collisions with CO2 is investigated as a function of initial pyrazine internal energy using a high-resolution laser transient absorption spectrometer. Experiments focus on energy exchanging collisions that result in excitation of rotations and translations in the ground vibrationless (0000) state of CO2. Highly vibrationally excited pyrazine (Evib=37 900 cm−1 or Evib=41 000 cm−1) is prepared via pulsed excitation at 266 nm or 246 nm, followed by rapid radiationless decay to the ground electronic state. The nascent CO2 rotational populations are measured by collecting the transient absorption of individual rovibrational lines at short times following the pyrazine excitation. The translational energies of CO2 recoiling from hot pyrazine are measured for numerous individual rotational levels. Energy dependent rate constants and probabilities are reported for both donor energies and results are compared with earlier studies using 248 nm excitation. These experiments reveal that for both donor energies, significant rotational and translational excitation of CO2 results from collisions with highly vibrationally excited pyrazine, as evidenced by the similarity in the observed rotational and translational distributions. Remarkably, however, the probabilities for the individual energy transfer pathways increase by as much as a factor of 3 for a 7% change in the pyrazine internal energy. The magnitudes and probabilities of energy transfer are described in terms of an energy transfer distribution function for the different donor molecule energies and implications for sequential quenching collisions are discussed.

The collisional deactivation of highly vibrationally excited pyrazine by a bath of carbon dioxide: Excitation of the infrared inactive (100), (020), and (0220) bath vibrational modes

The collisional quenching of highly vibrationally excited pyrazine, C4H4N2, by CO2 has been investigated using high resolution infrared transient absorption spectroscopy at a series of cell temperatures. Attention is focused on collisions which result in excitation of the Fermi-mixed bath vibrational states (1000) and (0200), along with the unmixed overtone bend state (0220). The vibrationally hot (Evib≈5 eV) pyrazine molecules are formed by 248 nm excimer laser pumping, followed by rapid radiationless decay to the ground electronic state. The nascent rotational and translational product state distributions of the CO2 molecules in each vibrationally excited state are probed at short times following the excitation of pyrazine. The temperature dependence of this process, along with the CO2 product state distributions and velocity recoils, strongly suggest that the vibrational excitation of CO2 is dominated by a long-range electrostatic interaction despite the fact that the dipole transition matrix elements connecting the CO2 ground state to the excited states vanish for the isolated molecule. The vibrational energy transfer is accompanied by very little rotational and translational excitation and displays the characteristic strong, inverse temperature dependence (probability of transfer increases with decreasing temperature) expected of energy transfer mediated by a long range attractive interaction. A number of possible explanations for this apparent anomaly are considered, of which energy transfer mediated by dipole/quadrupole forces appears to be the most consistent with the data.