Excited State Dissociation Energy Research Articles

Evaluation of Dissociation Energies of the Diatomic (NdO, FeH and BaF) Molecules

The ground and excited state dissociation energies are determined by curve fitting techniques using the five parameters Hulburt-Hirschfelder (H-H) function. The estimated dissociation energies are 7.33± 0.15eV, 2.90 ± 0.13 eV and 6.04± 0.12eV for NdO, FeH and BaF respectively. The computed values are in good agreement with the literature values. The nature of binding is discussed in the light of the percentage ionic\characters of these molecules.

Electronic spectroscopy of CoNe+ via mass-selected photodissociation

The CoNe(+) diatomic cation is produced by laser vaporization in a pulsed-nozzle source and studied with photodissociation spectroscopy at visible wavelengths. Vibronic structure is assigned to the (3)Π(2) ← (3)Δ(3) band system correlating to the Co(+)((3)P(2) ← (3)F(4)) + Ne asymptote. The origin band (13,529 cm(-1)) and a progression of 14 other vibrational bands are detected ending in the dissociation limit at 14,191 cm(-1). The excited state dissociation energy is therefore D(0)(') = 662 cm(-1), and an energetic cycle using this, the origin band energy, and the atomic transition produces a ground state dissociation energy of D(0)(") = 930 cm(-1). The excited state vibrational frequency is 116.1 cm(-1). A rotationally resolved study of the origin band confirms the electronic transition assignment and provides the bond distance of r(0)(") = 2.36 Å. The properties of CoNe(+) are compared to those of other CoRG(+) and MNe(+) complexes studied previously.

Time-dependent density functional study of excimers and exciplexes of organic molecules

Organic excimers of pyrene and benzene and an exciplex of styrene with trimethylamine are studied by time-dependent density functional theory (TD-DFT). The performance of the method was first benchmarked on noble gas excimers. Potential curves for low-lying excited states of Ne 2 and Kr 2 are computed using the hybrid functional BH-LYP and the semi-local B-LYP GGA functional. In order to include dispersion (van der Waals) effects, a previously developed R −6 type correction (DFT-D) was applied. The TD-DFT/BH-LYP results are in good agreement with those from CC2 or older MRD-CI treatments while pure GGAs fail miserably. The approach is then used to fully optimise the organic complexes in their ground and excited states. Excited state dissociation energies ( D 0) of 0.6, 1.2 and 0.8 eV are obtained for benzene and pyrene excimers and the organic exciplex of styrene and trimethylamine, respectively. For the latter system, the theoretical and experimental values agree to each other within 20%. For the excimers of pyrene and benzene, which have been measured in solution, the deviations are larger but within the range of expected solvent effects. In the case of the pyrene excimer, the TD-DFT/BH-LYP computed order of the low-lying excimer states is in agreement with that predicted by the simple exciton coupling model. The significant decrease of inter-molecular distances for all excited complexes compared to the ground state (0.07–0.4 Å) and the reasons for occurring rearrangements (favourable orbital interactions and charge transfer) are discussed.

Photodissociation spectroscopy of the Ca+–N2 complex

The weakly bound complex Ca+–N2 is prepared in a pulsed nozzle/laser vaporization cluster source and studied with mass-selected photodissociation spectroscopy. The chromophore giving rise to the electronic transition is the 2P←2S atomic transition of Ca+. The appearance of spin–orbit doublets in the vibrationally resolved spectrum, as expected for a 2∏r←2∑+ transition, confirms that the complex is linear. The electronic transition in the complex lies to the red of the atomic resonance line indicating that the complex is more strongly bound in the excited state than in the ground state. The vibrationally resolved spectrum contains progressions in the Ca+–N2 stretching mode and in a combination of this stretch with the N–N stretch. Extrapolation of the Ca+–N2 stretch determines the excited state dissociation energy to be D0′=6500±500 cm−1, and an energetic cycle determines the ground state value to be D0″=1755±500 cm−1 (5.02 kcal/mol). The 2∏r(2,0,0)←2∑+(0,0,0) vibronic transition has been rotationally resolved yielding the bond lengths: rCaN=2.75 Å and rNN=1.15 Å for the 2∑+ ground state; rCaN=2.48 Å and rNN=1.17 Å for the 2∏ excited state.

Accessing a low-lying bound electronic state of the alkali oxides, LiO and NaO, using laser induced fluorescence

Abstract The first laser based probe for the sodium and lithium monoxides is established. The Li(Na)+N2O reactions studied in a multiple collision entrainment mode produce the LiO and NaO ground X2Π and low-lying monoxide excited states. In contrast to the alkali halides, laser induced excitation spectroscopy confirms that the LiO and NaO B 2Π states, counter to recent predictions, are located at energies well below the ground state dissociation asymptote and, as predicted, possess significant binding energies. An assignment of the laser induced excitation spectra (LIF) for the B 2Π-X 2Π transitions of LiO in the region 3940–4300 A is based on a direct correlation with the observed chemiluminescence (CL) from the lowest level of the LiO B 2Π state ( ∼4000–7000 A ) and high quality ab initio calculations for the ground state. The self-consistent assignment of the observed LIF and CL spectra makes use of the complimentary extended progressions in the X 2Π (CL) and B 2Π (LIF) vibrational level structure which results from the significant shift of the B 2Π excited state potential relative to that of the ground state. The experimental data are consistent with an excited state vibrational frequency separation of order 130 cm−1, and T e ( B 2 Π) ≈ 26078 ± 800 cm −1 . The latter value, in correlation with the ground state dissociation energy of LiO, suggests a B 2Π excited state dissociation energy well in excess of 2000 cm−1. The radiative lifetimes of the lowest levels of the LiO B 2Π state, isoergic with the highest levels of the LiO ground state, are determined to be in excess of 600 ns. The corresponding NaO excitation spectra in the range 6680–7250 A also correlate well with ab initio calculations for the ground electronic state of NaO. Within this study, we provide optical signatures which one might consider to monitor LiO or NaO in process streams. In correlation with the observed chemiluminescence from B 2Π states of the higher alkali oxides KO, RbO, and CsO, this study suggests regions where one might probe their excitation spectra.

Photoionization spectroscopy of the In–N2 van der Waals complex

A vibrationally resolved electronic spectrum is observed for the metal atom van der Waals complex In–N2. Two electronic band systems are detected with mass resolved two-color photoionization spectroscopy. A lower energy system is observed slightly to the blue of the In ( 2D←P) atomic asymptote. It is characterized by a progression in the In–N2 stretching mode with a frequency of ω′e=76.7 cm−1. The higher energy system is slightly to the blue of the In (4P←2P) asymptote. It also exhibits a progression in the In–N2 stretch with a frequency of ω′e=87.7 cm−1. Extrapolation of the vibrational progressions leads to determination of the excited state dissociation energies. Energetic cycles based on the electronic transition energies, excited state dissociation energies, and atomic asymptotes lead to a determination of the ground state dissociation energy of D″0=1519 cm−1 (0.188 eV). A single-photon photoionization experiment determines the ionization potential to be 43 372 cm−1 (5.377 eV). This IP value, together with the atomic IP and the ground state neutral dissociation energy, yields a dissociation energy of D″0=4817 cm−1 (0.597 eV) for the In+–N2 ion–molecule complex.

Photodissociation spectroscopy of Mg+—Ar

Mg+—Ar ion—molecule complexes are produced in a pulsed supersonic nozzle cluster source. The complexes are mass selected and studied with laser photodissociation spectroscopy in a reflectron time-of-flight mass spectrometer system. An electronic transition assigned as X 2Σ+ → 2Π is observed with an origin at 31387 cm−1 (vac) for 24Mg+—Ar. The 24Mg+—Ar spectrum is characterized by a 15 member progression with a frequency (ω′e) of 272 cm−1. An extrapolation of this progression fixes the excited state dissociation energy (D′o) at 5552 cm−1. The corresponding ground-state value (D″o) is 1270 cm−1 (3.6 kcal/mol). The 2Π12,32 spin—orbit splitting is 76 cm−.

Photodissociation spectroscopy of Mg+–H2O and Mg+–D2O

Mg+–H2O ion–molecule complexes are produced in a pulsed supersonic nozzle cluster source. These complexes are mass selected and studied with laser photodissociation spectroscopy in a reflectron time-of-flight mass spectrometer system. An electronic transition assigned as 2B2←X 2A1 is observed with an origin at 28 396 cm−1. The spectrum has a prominent progression in the metal-H2O stretching mode with a frequency (ω′e) of 518.0 cm−1. An extrapolation of this progression fixes the excited state dissociation energy (D′0) at 15 787 cm−1. The corresponding ground state value (D″0) is 8514 cm−1 (24.3 kcal/mol). The solvated bending mode, and symmetric and asymmetric stretching modes of water are also active in the complex, as are the magnesium bending modes. A second electronic transition assigned as 2B1←X 2A1 is observed with an origin at 30 267 cm−1 and a metal stretch frequency for Mg+–H2O of 488.5 cm−1 (ΔG1/2). Spectra of both excited states are also observed for Mg+–D2O. Partially resolved rotational structure is analyzed for both isotopes, leading to the conclusion that the complex has a structure with C2v symmetry. This study was guided by ab initio calculations by Bauschlicher and co-workers, which provide accurate predictions of the electronic transition energies, vibrational constants, and dissociation energies.

Vibrational predissociation and intramolecular vibrational relaxation in dimethyl-s-tetrazine–argon complexes

The one- and two-argon van der Waals complexes of dimethyl tetrazine (DMT) have been investigated by means of laser-induced excitation and dispersed fluorescence spectroscopy. The band shifts D″0 − D′0 range from −23 to −31 cm−1 for DMT–Ar relative to DMT. The excited state dissociation energy of the DMT–Ar bond was estimated to be 270 ≤ D′0 ≤ 484 cm−1. In all levels with energy greater than the argon dissociation energy, predissociation is very fast. A dissociation rate constant kD = 3 × 109 s−1 has been determined for dissociation from the 6a1 level. This rate constant is an order of magnitude larger than that observed in tetrazine–argon. The dissociation rate constant of DMT–Ar2 from the 6a1 level is approximately 2 × 1010 s−1. Unlike aminotetrazine–Ar2, DMT–Ar2 shows no mode-selective variations in the branching ratios between the DMT and DMT–Ar photoproducts.

Photodissociation spectroscopy of Mg +H 2O

Mg +H 2O ion—molecule complexes are produced in a pulsed supersonic nozzle cluster source. These weakly bound complexes are mass selected and studied with laser photodissociation spectroscopy in a reflectron time-of-flight mass spectrometer system. An electronic transition assigned as 2B 2→X 2A 1 is observed with an origin at 28399 cm −1 (vac). The spectrum has a prominent progression in the metal—H 2O stretching mode with a frequency (ω′ e) of 517.1 cm −1. An extrapolation of this progression fixes the excited state dissociation energy ( D′ 0) at 16008 cm −1. The corresponding ground state value ( D″ 0) is 8732 cm −1 (25.0 kcal/mol). The solvated bending mode and asymmetric stretching mode of water are also active in the complex. A second electronic transition assigned as 2B 1←X 2A 1 is observed with an origin at 30386 cm −1 and a metal stretch frequency of 483.4 cm −1. This study was guided by ab initio calculations by Bauschlicher and co-workers, which provide accurate predictions of the electronic transition energies, vibrational constants and dissociation energies.

Photodissociation spectroscopy of Mg + 2

Mg +CO 2 ion—molecule complexes are produced in a pulsed supersonic nozzle cluster source. These weakly bound complexes are mass-selected and studied with laser photodissociation spectroscopy in a reflectron time-of-flight mass spectrometer system. An electronic transition assigned as 2Π←X 2Σ + is observed with an origin at 29625 cm −1 (vac). The spectrum is characterized by a six-member progression in the metal—CO 2 stretching mode with a frequency (ω′ e) of 381.8 cm −1. An extrapolation of this progression fixes the excited state dissociation energy (D′ 0) at 11194 cm −1. The corresponding ground state value (D″ 0) is 5150 cm −1 (14.7 kcal/mol). The 2Π 1 2 , 3 2 spin—orbit splitting is 56 cm −1. These studies were guided by ab initio calculations by Bauschlicher and co-workers, which provide accurate predictions of the electronic transition energy, vibrational constants and dissociation energy.

Electronic spectroscopy of silver dimer rare gas complexes

Vibrationally resolved electronic spectra are reported for the metal dimer-rare gas complexes Ag2–Ar and Ag2–Kr. These spectra are obtained using resonant two-photon photoionization in the energy region near the Ag2 B←X electronic transition (280–285 nm). Both complexes exhibit extensive activity in three vibrational modes, making it possible to determine vibrational constants, anharmonicities, and cross-mode couplings. An unusual cancellation of factors results in the Kr complex (ω′e =72.6 cm−1) having nearly the same metal-rare gas stretching frequency as the Ar complex (ωe=73.9 cm−1). Progressions extending over a significant range of the excited state potential surfaces make it possible to derive the excited state dissociation energies (D′0=755 and 1205 cm−1 for Ar and Kr, respectively). Combination with the red-shifted electronic state origins yields the corresponding ground state dissociation energies (D■0=275 and 394 cm−1 for Ar and Kr, respectively). Potential energy surfaces are investigated for excited and ground states of both complexes.

Vibronic spectrum of XeKr and determination of ground and excited state dissociation energies

A fluorescence excitation spectrum of the rare gas dimer XeKr has been recorded at ≈ 123 nm. The analysis of 18 closely-spaced vibronic bands leads to a dissociation energy of 150.5±0.2 cm −1 for the ground state and to estimates of the internuclear distance and dissociation energy of the excited state.

Potential curves from continuous spectra: He2X1Σg+andA1Σu+

Continuous electronic spectra of diatomic molecules can often be related to the associated potential curves. It is demonstrated that the local nature of continuous spectra may greatly facilitate inversion of the spectra to obtain ground and excited state potential curves. All of the observed continuous spectra related to transitions between the X and A states of the helium diatomic molecule are re-examined with the goal of determining accurate potential curves for the two states. It is shown that the excited state dissociation energy is 2·50±0·03 eV and that the MDD-1 potential proposed by Bruch and McGee is the preferred choice to represent the ground state interaction. Also, the mean cross section at 300 K for the destruction of a 21 S helium atom by collision-induced emission is evaluated and found to be 2·8 × 10-20 cm2 in agreement with the measurements of Phelps.