UV induced local heating effects in TiO2 nanocrystals

When isolated TiO(2) nanocrystals are subjected to UV light at 77 K and pressures below 10(-6) mbar, trapping of photogenerated hole centers occurs on the surface of the nanocrystals and can be tracked by time-resolved electron paramagnetic resonance spectroscopy. Irrespective of the selected UV irradiance used, the maximum concentration of trapped charges was found to be constant for a given number of nanocrystals ( approximately 10(15)) and corresponds to one electron-hole pair per particle. On a time scale of seconds to minutes the dynamics for the trapping process depend on the number of photons with supra band gap energy. A local temperature rise of the TiO(2) nanocrystals was observed for irradiances above 1.55 mW cm(-2) (10(15) photons cm(-2) s(-1)). This is attributed to enhanced nonradiative recombination of photogenerated charge carriers via heat production and points to a substantial contribution of thermal chemistry in photocatalytic reaction cycles.     

Charge recombination fluorescence in photosystem I reaction centers from Chlamydomonas reinhardtii

The fluorescence kinetics of photosystem I core particles from Chlamydomonas reinhardtii have been measured with picosecond resolution in order to test a previous hypothesis suggesting a charge recombination mechanism for the early electron-transfer steps and the fluorescence kinetics (Müller et al. Biophys. J. 2003, 85, 3899-3922). Performing global target analyses for various kinetic models on the original fluorescence data confirms the "charge recombination" model as the only acceptable one of the models tested while all of the other models can be excluded. The analysis allowed a precise determination of (i) the effective charge separation rate constant from the equilibrated reaction center excited state (438 ns(-1)) confirming our previous assignment based on transient absorption data (Müller et al. Biophys. J. 2003, 85, 3899-3922), (ii) the effective charge recombination rate constant back to the excited state (52 ns(-1)), and (iii) the intrinsic secondary electron-transfer rate constant (80 ns(-1)). The average energy equilibration lifetime core antenna/RC is about 1 ps in the "charge recombination" model, in agreement with previous transient absorption data, vs the 18-20 ps energy transfer lifetime from antenna to RC within "transfer-to-the-trap-limited" models. The apparent charge separation lifetime in the recombination model is about three times faster than in the "transfer-to-the-trap-limited" model. We conclude that the charge separation kinetics is trap-limited in PS I cores devoid of red antenna states such as in C. reinhardtii.     

Water Activity Regulates the Q(A)(-) to Q(B) electron transfer in photosynthetic reaction centers from Rhodobacter sphaeroides

We report on the effects of water activity and surrounding viscosity on electron transfer reactions taking place within a membrane protein: the reaction center (RC) from the photosynthetic bacterium Rhodobacter sphaeroides. We measured the kinetics of charge recombination between the primary photoxidized donor (P(+)) and the reduced quinone acceptors. Water activity (aW) and viscosity (eta) have been tuned by changing the concentration of cosolutes (trehalose, sucrose, glucose, and glycerol) and the temperature. The temperature dependence of the rate of charge recombination between the reduced primary quinone, Q(A)(-), and P(+) was found to be unaffected by the presence of cosolutes. At variance, the kinetics of charge recombination between the reduced secondary quinone (Q(B)(-)) and P(+) was found to be severely influenced by the presence of cosolutes and by the temperature. Results collected over a wide eta-range (2 orders of magnitude) demonstrate that the rate of P(+)Q(B)(-) recombination is uncorrelated to the solution viscosity. The kinetics of P(+)Q(B)(-) recombination depends on the P(+)Q(A)(-)Q(B) <--> P(+)Q(A)Q(B)(-) equilibrium constant. Accordingly, the dependence of the interquinone electron transfer equilibrium constant on T and aW has been explained by assuming that the transfer of one electron from Q(A)(-) to Q(B) is associated with the release of about three water molecules by the RC. This implies that the interquinone electron transfer involves at least two RC substates differing in the stoichiometry of interacting water molecules.     

Temperature dependence of charge recombination in Heliobacillus mobilis

Transient absorption difference spectroscopy was used to study the temperature dependence of the P798+ decay kinetics in heliobacteria. For membrane samples, two components were obtained from the fitting of kinetic traces in the temperature range of 4-29 degrees C. A 3-9 ms component representing the cytochrome (cyt) c oxidation has an activation energy of 33.0 +/- 2.8 kJ/mol. A 12-22 ms component representing either P798+FX- or P798+FA/B- recombination has an activation energy of 15.3 +/- 2.4 kJ/mol. In isolated reaction centers (RC), only one 14 ms component due to P798+FX- recombination was obtained in this temperature range. The Arrhenius plot shows that the recombination rate of this P798+FX- state is temperature independent in the near room temperature range. For RC in the temperature range of 60-298 K, a 12-15 ms decay was obtained at temperatures greater than 240 K. Biphasic decay traces (12-15 ms and 2-4 ms components) were obtained at temperatures between 170 K and 230 K. Only one 2-4 ms component was found at temperatures lower than 160 K. The gradual switchover from the 12-15 ms to the 2-4 ms component upon cooling may indicate the shift of the P798+FX- recombination state to a state that is prior to P798+FX-, although other interpretations can not be excluded. The absorption difference spectrum (delta A @ 160 K - delta A @ 240 K) in the blue region shows a positive amplitude below 405 nm and a negative amplitude above 405 nm implying that the 2-4 ms decay component may be due to the recombination of P798+A1-, where A1 is a quinone-type acceptor.     

Reversible bridge-mediated excited-state symmetry breaking in stilbene-linked DNA dumbbells

The excited-state behavior of synthetic DNA dumbbells possessing stilbenedicarboxamide (Sa) linkers separated by short A-tracts or alternating A-T base-pair sequences has been investigated by means of fluorescence and transient absorption spectroscopy. Electronic excitation of the Sa chromophores results in conversion of a locally excited state to a charge-separated state in which one Sa is reduced and the other is oxidized. This symmetry-breaking process occurs exclusively via a multistep mechanism-hole injection followed by hole transport and hole trapping-even at short distances. Rate constants for charge separation are strongly distance-dependent at short distances but become less so at longer distances. Disruption of the A-tract by inversion of a single A-T base pair results in a pronounced decrease in both the rate constant and efficiency of charge separation. Hole trapping by Sa is highly reversible, resulting in rapid charge recombination that occurs via the reverse of the charge separation process: hole detrapping, hole transport, and charge return to regenerate the locally excited Sa singlet state. These results differ in several significant respects from those previously reported for guanine or stilbenediether as hole traps. Neither charge separation nor charge recombination occur via a single-step superexchange mechanism, and hole trapping is slower and detrapping faster when Sa serves as the electron donor. Both the occurrence of symmetry breaking and reversible hole trapping by a shallow trap in a DNA-based system are without precedent.     

Picosecond dynamics of proton transfer of a 7-hydroxyflavylium salt in aqueous-organic solvent mixtures

The intermediacy of the geminate base-proton pair (A*···H(+)) in excited-state proton-transfer (ESPT) reactions (two-step mechanism) has been investigated employing the synthetic flavylium salt 7-hydroxy-4-methyl-flavylium chloride (HMF). In aqueous solution, the ESPT mechanism involves solely the excited acid AH(+)* and base A* forms of HMF as indicated by the fluorescence spectra and double-exponential fluorescence decays (two species, two decay times). However, upon addition of either 1,4-dioxane or 1,2-propylene glycol, the decays become triple-exponential with a term consistent with the presence of the geminate base-proton pair A*···H(+). The geminate pair becomes detectable because of the increase in the recombination rate constant, k(rec), of (A*···H(+)) with increasing the mole fraction of added organic cosolvent. Because the two-step ESPT mechanism splits the intrinsic prototropic reaction rates (deprotonation of AH(+)*, k(d), and recombination, k(rec), of A*···H(+)) from the diffusion controlled rates (dissociation, k(diss), and formation, k(diff)[H(+)], of A*···H(+)), the experimental detection of the geminate pair provides a wealth of information on the proton-transfer reaction (k(d) and k(rec)) as well as on proton diffusion/migration (k(diss) and k(diff)).     

Computer investigations on the asymptotic behavior of the rate coefficient for the annihilation reaction A + A → product and the trapping reaction in three dimensions

We have performed intensive computer simulations of the irreversible annihilation reaction: A + A → C + C and of the trapping reaction: A + B → C + B for a variety of three-dimensional fluids composed of identical spherical particles. We have found a significant difference in the asymptotic behavior of the rate coefficients for these reactions. Both the rate coefficients converge to the same value with time t going to infinity but the convergence rate is different: the O(t(-1/2)) term for the annihilation reaction is higher than the corresponding term for the trapping reaction. The simulation results suggest that ratio of the terms is a universal quantity with the value equal to 2 or slightly above. A model for the annihilation reaction based on the superposition approximation predicts the difference in the O(t(-1/2)) terms, but overestimates the value for the annihilation reaction by about 30%. We have also performed simulations for the dimerization process: A + A → E, where E stands for a dimer. The dimerization decreases the reaction rate due to the decrease in the diffusion constant for A. The effect is successfully predicted by a simple model.     

Hole mobility and lifetime in a smectic liquid crystalline photoconductor of a 2-phenylnaphthalene derivative

We have investigated hole transport properties in the smectic mesophases of a 2-phenylnaphthalene derivative 6-(4'-octylphenyl)-2-dodecyloxynaphthalene in detail by using time-of-flight technique. The transient photocurrents were measured in liquid-crystal cells with various thickness from 5 to 700 microm. They were well defined and nondispersive in the smectic A (SmA) phase up to 500 microm and in the smectic B (SmB) phase within the entire thickness employed, while they exhibited an exponential decay in the SmA phase at 700 microm. The mobilities in the SmA and SmB phases were constant in each mesophase irrespective of the cell thickness, and were 2.5 x 10(-4) and 1.7 x 10(-3) cm2V s, respectively. The hole lifetimes were determined to be 10 ms and longer than 5 ms for the SmA and SmB phases, respectively. We discuss the origin of these lifetimes from the two points of view, i.e., hole trapping by a trace amount of existing impurities and recombination with negative ionic charges. We conclude that impurities are mainly responsible for the present hole lifetime test.     

Electron transfer kinetics in photosynthetic reaction centers embedded in polyvinyl alcohol films

The coupling between electron transfer and protein dynamics has been studied at room temperature in isolated reaction centers (RCs) from the photosynthetic bacterium Rhodobacter sphaeroides by incorporating the protein in polyvinyl alcohol (PVA) films of different water/RC ratios. The kinetic analysis of charge recombination shows that dehydration of RC-containing PVA films causes reversible, inhomogeneous inhibition of electron transfer from the reduced primary quinone acceptor (Q(A)(-)) to the secondary quinone Q(B). A more extensive dehydration of solid PVA matrices accelerates electron transfer from Q(A)(-) to the primary photooxidized electron donor P(+). These effects indicate that incorporation of RCs into dehydrated PVA films hinders the conformational dynamics gating Q(A)(-) to Q(B) electron transfer at room temperature and slows down protein relaxation which stabilizes the primary charge-separated state P(+)Q(A)(-). A comparison with analogous effects observed in trehalose-coated RCs suggests that protein motions are less severely reduced in PVA films than in trehalose matrices at comparable water/RC ratios.     

Solvent Cage Effects in Organocobalt Corrinoid Chemistry: Thermal Homolysis of alpha- and beta-(Cyanomethyl)cobinamides(1)

The equilibrium constant for the thermal isomerization of the diastereomeric alpha- and beta-(cyanomethyl)cobinamides (NCCH(2)Cbi(+)'s) has been measured over the temperature range 70-95 degrees C. Although the beta diastereomer is the thermodynamically more stable isomer, it is favored by the entropy change, but disfavored by the enthalpy change. In the presence of >/=5 x 10(-)(3) M concentration of the radical trap 4-hydroxy-2,2,6,6,-tetramethylpiperidinyloxy (4-HTEMPO), thermolysis of either isomer leads to cob(II)inamide and the trapped NCCH(2)(*) radical (NCCH(2)-4-HTEMPO) in high yield and no isomerization can be detected. The kinetics of the 4-HTEMPO-trapped thermal homolysis of alpha- and beta-NCCH(2)Cbi(+) have been studied in anaerobic glycerol/water mixtures of varying viscosity. The observed first-order rate constants for thermolysis show the expected inverse dependence on viscosity indicating that the process is at least partially diffusion controlled. From these data, the primary rate constant, k(1), for carbon-cobalt bond homolysis and the ratio of the rate constants for in-cage recombination and diffusional separation (k(c)/k(d)) can be extracted. The enthalpies of activation for Co-C bond homolysis are identical (29.0 +/- 0.3 kcal mol(-)(1)) while the entropy of activation is 2-fold higher for the alpha diastereomer. In water, the fractional cage efficiencies, F(c), are quite small (0.12 +/- 0.01, alpha; 0.049 +/- 0.008, beta) and invariant for each complex in the temperature range 75-95 degrees C. Assuming that the rate constant for diffusional separation of the caged radical pairs is the same for both isomers, the ratio of the in-cage recombination rate constants, k(c)(alpha)/k(c)(beta), can be calculated to be 2.6 +/- 0.6. This surprising kinetic preference for the alpha diastereomer results from enthalpic stabilization of the recombination transition state for the alpha diastereomer, since the beta diastereomer is entropically favored.     

Reaction of (carbonylimido)sulfur(IV) derivatives with TAS-fluoride, (Me2N)3S+Me3SiF2-

In the reaction of TAS-fluoride, (Me2N)3S+Me3SiF2-, with carbonyl sulfur difluoride imides RC(O)NSF2 (R = F, CF3), C-N bond, cleavage is observed, and TAS+RC(O)F2- and NSF are the final products. From TASF and RC(O)NS(CF3)F, the salts TAS+RC(O)NS(CF3)F2- (R = F (14), CF3 (15)), with psi-pentacoordinate sulfur centers in the anions, are formed. An X-ray structure investigation of 14 shows that the fluorine atoms occupy axial positions and CF3, NC(O)F, and the sulfur lone pair occupy equatorial positions of the trigonal bipyramid. The -C(O)F group lies in the equatorial plane with the CO bond synperiplanar to the SN bond. According to B3LYP calculations, this structure corresponds to a global minimum and the expected axial orientation of the -C(O)F group represents a transition state. Calculations for the unstable FC(O)NSF3- anion show a different geometry. The -C(O)F group deviates 40 degrees from axial orientation, and the equatorially bonded fluorine is, in contrast to the -CF3 group in 14, syn positioned.     

Dissociative electron-ion recombination of the interstellar species protonated glycolaldehyde, acetic acid, and methyl formate

Recently, methyl formate, glycolaldehyde, and acetic acid have been detected in the Interstellar Medium, ISM. The rate constants, α(e), for dissociative electron-ion recombination of protonated gycolaldehyde, (HOCH(2)CHO)H(+), and protonated methyl formate, (HCOOCH(3))H(+), have been determined at 300 K in a variable temperature flowing afterglow using a Langmuir probe to obtain the electron density. The recombination rate constants at 300 K are 3.2 × 10(-7) cm(3) s(-1) for protonated methyl formate and 7.5 × 10(-7) cm(3) s(-1) for protonated glycolaldehyde. The recombination rate constant of protonated acetic acid could not be directly measured, but it appears to have a rate constant, α(e), on the 10(-7) cm(3) s(-1) scale. Several high- and low-temperature measurements for protonated methyl formate were made. In addition, an α(e) measurement at 220 K for protonated glycolaldehyde was performed. The astrochemical implications of the rates of recombination, α(e), and protonation routes are discussed.     

Theoretical designs for germaacetylene (RC≡GeR'): a new target for synthesis

The effect of substitution on the potential energy surfaces of RC≡GeR (R = F, H, OH, CH(3), SiH(3), Tbt, Ar*, SiMe(SitBu(3))(2) and SiiPrDis(2)) was explored using density functional theories (B3LYP/LANL2DZdp and B3PW91/6-31G(d)). Our theoretical studies indicate that all the triply bonded RC≡GeR species prefer to adopt trans-bent geometry, which is in agreement with the theoretical model (mode (B)). Additionally, we show that the stabilities of the RC≡GeR species bearing smaller substituents (R = F, H, OH, CH(3) and SiH(3)) decrease in the order R(2)C=Ge: > RC≡GeR > :C=GeR(2). On the other hand, the triply bonded RC≡GeR molecules with bulkier substituents (R = Tbt, Ar*, SiMe(SitBu(3))(2), SiiPrDis(2)) were found to possess the global minimum on the singlet potential energy surface and are both kinetically and thermodynamically stable. That is to say, both electronic and steric effects of bulky substituents play a crucial role in making triply bonded germaacetylenes (RC≡GeR) an interesting synthetic target.     

Excited-state proton transfer: indication of three steps in the dissociation and recombination process

A femtosecond pump-probe, with approximately 150 fs resolution, as well as time-correlated single photon counting with approximately 10 ps resolution techniques are used to probe the excited-state intermolecular proton transfer from HPTS to water. The pump-probe signal consists of two ultrafast components (approximately 0.8 and 3 ps) that precede the relatively slow (approximately 100 ps) component. From a comparative study of the excited acid properties in water and methanol and of its conjugate base in basic solution of water, we propose a modified mechanism for the ESPT consisting of two reactive steps followed by a diffusive step. In the first, fast, step the photoacid dissociates at about 10 ps to form a contact ion pair RO-*...H3O+. The contact ion pair recombines efficiently to re-form the photoacid with a recombination rate constant twice as large as the dissociation rate constant. The first-step equilibrium constant value is about 0.5 and thus, at short times, <10 ps, only approximately 30% of the excited photoacid molecules are in the form of the conjugated base-proton contact ion pair. In the second, slower, step, of about 100 ps, the proton is separated by at least one water molecule from the conjugate base RO-. The separated proton and the conjugated base can recombine geminately as described by our previous diffusion-assisted model. The new two-step reactive model predicts that the population of the ROH form of HPTS will decrease with two time constants and the RO- population will increase by the same time constants. The proposed model fits the experimental data of this study as well as previous published experimental data.     

Degenerate electron exchange reaction of n-alkane radical cations in solution

The degenerate electron exchange (DEE) reaction involving radical cations (RCs) of n-nonane, n-dodecane, and n-hexadecane in n-hexane solution was studied over the temperature range 253-313 K using the method of time-resolved magnetic field effect in recombination fluorescence of spin-correlated radical ion pairs. In the dilute solutions the rate constant of DEE was found to be 200 times slower than the diffusion limit. Using n-nonane as an example, we showed that two reasons are responsible for the low value of the RC self-exchange rate: (1) conformational variability of molecules and RCs and (2) the activation barrier of DEE reaction. The calculations of the reaction enthalpy performed by the B3LYP/6-31G(d) method indicated that electron transfer can be effective only upon collision of RC with a neutral molecule either in the all-trans conformation or in the conformation differing from the latter by rotation of the end ethyl fragment. The activation barrier of the DEE reaction was estimated using the reorganization energy of the internal degrees of freedom calculated at the B3LYP level and was found to be about 6 kcal/mol. A possible influence of the interaction between RC and a neutral molecule in an encounter complex on DEE rate constant is also discussed.     

Enhanced Capacitive Characteristics of Activated Carbon by Secondary Activation

The effect of the improvement of commercial activated carbon(AC) on its specific capacitance and high rate capability of double layer(dl) charging/discharging process has been studied. The improvement of AC was carried out via a secondary activation under steam in the presence of catalyst NiCl2, and the suitable condition was found to be a heat treatment at about 875 ℃ for 1 h. Under those conditions, the discharge specific capacitance of the improved AC increases up to 53. 67 F/g, showing an increase of about 25% as compared with that of as-received AC. The good rectangular-shaped voltammograms and A.C. impedance spectra prove that the high rate capability of the capacitor made of the improved AC is enhanced significantly. The capacitance resistance (RC) time constant of the capacitor containing the improved AC is 1.74 s, which is much lower than that of the one containing as-received AC(an RC value of 4. 73 s). It is noted that both kinds of AC samples show a similar specific surface area and pore size distribution, but some changes have taken place in the carbon surface groups, especially a decrease in the concentration of surface carbonyl groups after the improvement, which have been verified by means of X-photoelectron spectroscopy. Accordingly, it is suggested that the decrease in the concentration of surface carbonyl groups for the improved AC is beneficial to the organic electrolyte ion penetrating into the pores, thus leading to the increase in both the specific capacitance and high rate capability of the supercapacitor.     

Photo-induced proton-transfer cycle of 2-naphthol in faujasite zeolitic nanocavities

The excited-state deprotonation and ground-state reprotonation of a 2-naphthol molecule encapsulated in the zeolitic nanocavity of NaX have been studied by measuring static and time-resolved spectra of fluorescence and reflectance. The excited molecule undergoes enol dissociation within 300 ps to form an isolated ion pair, which undergoes geminate recombination in 1200 ps or separation to produce the anionic species of 2-naphtholate on the time scale of 2500 ps. Ground-state reprotonation, controlled by the diffusion rate of a proton, is then followed in 0.8 ms with an activation energy of 13 kJ mol(-1).     

The fluence rate determines the synergistic interaction of UV radiation and heat for mitotic recombination and cell inactivation in yeasts

The significance of the UV fluence rate for the synergistic interaction of UV light (254 nm) and heat was demonstrated for the frequency of mitotic recombination in a wild-type diploid yeast of Saccharomyces cerevisiae (strain T1) and for cell inactivation of two wild-type diploid yeast of S. cerevisiae (strains T1, XS800). It was shown for mitotic recombination that a decrease in the intensity of UV exposure results in the necessity of decreasing the temperature at which UV irradiation occurs to provide the same value of the synergistic enhancement ratio. For cell inactivation, there was a specific temperature maximizing the synergistic effect for any constant fluence rate and the temperature range, synergistically increasing the inactivation effect of UV radiation, should be shifted to lower temperatures with a decrease in the fluence rate. To interpret the results observed, a simple mathematical model of the synergistic interaction was applied. The model suggests that the synergistic interaction of UV light and hyperthermia is expected to result from some additional effective damages arising from the interaction of some sublesions induced by both agents. On the basis of data obtained, it was supposed that the synergistic interaction of these factors might take place at small intensities of UV light and temperatures existing in the biosphere. In other words, for a long duration of interaction, which is important for problems of UV light protection and health effects, one can expect a synergistic interaction of this factor with environmental heat or physiological temperatures and thereby an increase in their inactivating and genetic consequences.     

Slow dynamics in folded and unfolded states of an SH3 domain.

(15)N relaxation dispersion experiments were applied to the isolated N-terminal SH3 domain of the Drosophila protein drk (drkN SH3) to study microsecond to second time scale exchange processes. The drkN SH3 domain exists in equilibrium between folded (F(exch)) and unfolded (U(exch)) states under nondenaturing conditions in a ratio of 2:1 at 20 degrees C, with an average exchange rate constant, k(ex), of 2.2 s(-1) (slow exchange on the NMR chemical shift time scale). Consequently a discrete set of resonances is observed for each state in NMR spectra. Within the U(exch) ensemble there is a contiguous stretch of residues undergoing conformational exchange on a micros/ms time scale, likely due to local, non-native hydrophobic collapse. For these residues both the F(exch) <--> U(exch) conformational exchange process and the micros/ms exchange event within the U(exch) state contribute to the (15)N line width and can be analyzed using CPMG-based (15)N relaxation dispersion measurements. The contribution of both processes to the apparent relaxation rate can be deconvoluted numerically by combining the experimental (15)N relaxation dispersion data with results from an (15)N longitudinal relaxation experiment that accurately quantifies exchange rates in slow exchanging systems (Farrow, N. A.; Zhang, O.; Forman-Kay, J. D.; Kay, L. E. J. Biomol. NMR 1994, 4, 727-734). A simple, generally applicable analytical expression for the dependence of the effective transverse relaxation rate constant on the pulse spacing in CPMG experiments has been derived for a two-state exchange process in the slow exchange limit, which can be used to fit the experimental data on the global folding/unfolding transition. The results illustrate that relaxation dispersion experiments provide an extremely sensitive tool to probe conformational exchange processes in unfolded states and to obtain information on the free energy landscape of such systems.     

Determination of the rate constant for the NH2(X2B1) + NH2(X2B1) recombination reaction with collision partners He, Ne, Ar, and N2 at low pressures and 296 K. Part 1

The recombination rate constant for the NH(2)(X(2)B(1)) + NH(2)(X(2)B(1)) → N(2)H(4)(X(1)A(1)) reaction in He, Ne, Ar, and N(2) was measured over the pressure range 1-20 Torr at a temperature of 296 K. The NH(2) radical was produced by 193 nm laser photolysis of NH(3) dilute in the third-body gas. The production of NH(2) and the loss of NH(3) were monitored by high-resolution continuous-wave absorption spectroscopy: NH(2) on the (1)2(21) ← (1)3(31) rotational transition of the (0,7,0)A(2)A(1) ← (0,0,0) X(2)B(1) vibronic band and NH(3) on either inversion doublet of the (q)Q(3)(3) rotational transition of the ν(1) fundamental. Both species were detected simultaneously following the photolysis laser pulse. The broader Doppler width of the NH(2) spectral transition allowed temporal concentration measurements to be extended up to 20 Torr before pressure broadening effects became significant. Fall-off behavior was identified and the bimolecular rate constants for each collision partner were fit to a simple Troe form defined by the parameters, k(0), k(inf), and F(cent). This work is the first part of a two part series in which part 2 will discuss the measurements with more efficient energy transfer collision partners CH(4), C(2)H(6), CO(2), CF(4), and SF(6). The pressure range was too limited to extract any new information on k(inf), and k(inf) was taken from the theoretical calculations of Klippenstein et al. (J. Phys. Chem A 2009, 113, 10241) as k(inf) = 7.9 × 10(-11) cm(3) molecule(-1) s(-1) at 296 K. The individual Troe parameters were: He, k(0) = 2.8 × 10(-29) and F(cent) = 0.47; Ne, k(0) = 2.7 × 10(-29) and F(cent) = 0.34; Ar, k(0) = 4.4 × 10(-29) and F(cent) = 0.41; N(2), k(0) = 5.7 × 10(-29) and F(cent) = 0.61, with units cm(6) molecule(-2) s(-1) for k(0). In the case of N(2) as the third body, it was possible to measure the recombination rate constant for the NH(2) + H reaction near 20 Torr total pressure. The pure three-body recombination rate constant was (2.3 ± 0.55) × 10(-30) cm(6) molecule(-2) s(-1), where the uncertainty is the total experimental uncertainty including systematic errors at the 2σ level of confidence.