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Photoinitiated H2CO unimolecular decomposition: Accessing H+HCO products via S0 and T1 pathways (2000)

Valachovic, L. R. ; Tuchler, M. F. ; Dulligan, M. ; [et al.]

College Park, Md. : American Institute of Physics (AIP)

The Journal of Chemical Physics 112 (2000), S. 2752-2761

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ISSN: 1089-7690

Source: AIP Digital Archive

Topics: Physics , Chemistry and Pharmacology

Notes: The photoinitiated unimolecular decomposition of formaldehyde via the H+HCO radical channel has been examined at energies where the S0 and T1 pathways both participate. The barrierless S0 pathway has a loose transition state (which tightens somewhat with increasing energy), while the T1 pathway involves a barrier and therefore a tight transition state. The product state distributions which derive from the S0 and T1 pathways differ qualitatively, thereby providing a means of discerning the respective S0 and T1 contributions. Energies in excess of the H+HCO threshold have been examined throughout the range 1103≤E†≤2654 cm−1 by using two complementary experimental techniques; ion imaging and high-n Rydberg time-of-flight spectroscopy. It was found that S0 dominates at the low end of the energy range. Here, T1 participation is sporadic, presumably due to poor coupling between zeroth-order S1 levels and T1 reactive resonances. These T1 resonances have small decay widths because they lie below the T1 barrier. Alternatively, at the high end of the energy range, the T1 pathway dominates, though a modest S0 contribution is always present. The transition from S0 dominance to T1 dominance occurs over a broad energy range. The most reliable value for the T1 barrier (1920±210 cm−1) is given by the recent ab initio calculations of Yamaguchi et al. It lies near the center of the region where the transition from S0 dominance to T1 dominance takes place. Thus, the present results are consistent with the best theoretical calculations as well as the earlier study of Chuang et al., which bracketed the T1 barrier energy between 1020 and 2100 cm−1 above the H+HCO threshold. The main contribution of the present work is an experimental demonstration of the transition from S0 to T1 dominance, highlighting the sporadic nature of this competition. © 2000 American Institute of Physics.

Type of Medium: Electronic Resource

URL: http://dx.doi.org/10.1063/1.480849

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Photofragment imaging of HNCO decomposition: Angular anisotropy and correlated distributions (1997)

Sanov, A. ; Droz-Georget, Th. ; Zyrianov, M. ; [et al.]

College Park, Md. : American Institute of Physics (AIP)

The Journal of Chemical Physics 106 (1997), S. 7013-7022

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ISSN: 1089-7690

Source: AIP Digital Archive

Topics: Physics , Chemistry and Pharmacology

Notes: Photodissociation of jet-cooled isocyanic acid has been examined by photofragment ion imaging of H(D) from H(D)NCO and CO from HNCO, and by laser induced fluorescence (LIF) of NH(a 1Δ) from HNCO. Only modest recoil anisotropy is observed in the H+NCO channel at 243.1 nm (β=−0.13±0.05), while the D+NCO channel at approximately the same wavelength reveals no anisotropy (β=0.00±0.05), confirming that the dissociation of H(D)NCO from the opening of the H(D) channel proceeds via vibrational predissociation on the S0(1A) surface. In contrast, substantial anisotropy (β=−0.66±0.08) is observed in the NH(a 1Δ)+CO channel at 230.1 nm, but this value can correspond to dissociation on either S0 or S1. The photolysis region between 243 and 230 nm thus appears important in providing clues to the dissociation mechanism and the competition between different potential energy surfaces. At 217.6 nm, product state distributions exhibit clear dynamical biases. CO is produced in both ν=0 and ν=1, while NH(a 1Δ) distributions correlated with different rovibrational levels of CO, although different in shape, are always cold, consistent with the global NH distribution measured by LIF. The NH distributions indicate dissociation on S1(1A′′), and can be described by Franck–Condon mapping of transition state wave functions in the HNC bending coordinate without additional torque, implying little anisotropy in the potential along that coordinate. On the other hand, a larger torque is manifest in the CO rotational distribution. Although at 217.6 nm the dissociation is likely to be dominated by decomposition on S1, competition with radiationless decay is still manifest. From analysis of the CO photofragment velocity distribution at 230.1 nm, the NH(a 1Δ)+CO dissociation threshold is determined at 42 765±25 cm−1. © 1997 American Institute of Physics.

Type of Medium: Electronic Resource

URL: http://dx.doi.org/10.1063/1.473724

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Competition between singlet and triplet channels in the photoinitiated decomposition of HNCO (1997)

Zyrianov, M. ; Droz-Georget, Th. ; Reisler, H.

College Park, Md. : American Institute of Physics (AIP)

The Journal of Chemical Physics 106 (1997), S. 7454-7457

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ISSN: 1089-7690

Source: AIP Digital Archive

Topics: Physics , Chemistry and Pharmacology

Notes: The spin-forbidden channel, NH(X 3∑−)+CO, has been observed directly in the photodissociation of jet-cooled HNCO following S1←S0 excitation. The NH(X 3∑−) yield spectrum is obtained in the energy regimes near the thresholds to H+NCO and NH(a 1Δ)+CO channels. The spectrum is similar to the NCO yield spectrum when direct dissociation on S1 is not significant. At photolysis energies 〉43 400 cm−1, state specific differences between the NH(X 3∑−), NH(a 1Δ) and NCO yield spectra are observed, and at energies 〉44 000 cm−1 all structure in the NH(X 3∑−) yield spectrum is lost, while the NH(a 1Δ) yield spectrum remains structured. The results are interpreted in terms of the different time scales for intersystem crossing and dissociation.© 1997 American Institute of Physics.

Type of Medium: Electronic Resource

URL: http://dx.doi.org/10.1063/1.473705

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Competitive photodissociation channels in jet-cooled HNCO: Thermochemistry and near-threshold predissociation (1996)

Zyrianov, M. ; Droz-Georget, Th. ; Sanov, A. ; [et al.]

College Park, Md. : American Institute of Physics (AIP)

The Journal of Chemical Physics 105 (1996), S. 8111-8116

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ISSN: 1089-7690

Source: AIP Digital Archive

Topics: Physics , Chemistry and Pharmacology

Notes: The photoinitiated unimolecular decomposition of jet-cooled HNCO has been studied following S1(1A″)←S0(1A′) excitation near the thresholds of the spin-allowed dissociation channels: (1) H(2S)+NCO(X2Π) and (2) NH(a1Δ)+CO(X1Σ+), which are separated by 4470 cm−1. Photofragment yield spectra of NCO(X2Π) and NH (a1Δ) were obtained in selected regions in the 260–220 nm photolysis range. The NCO(X2Π)yield rises abruptly at 38 380 cm−1 and the spectrum exhibits structures as narrow as 0.8 cm−1 near the threshold. The linewidths increase only slowly with photolysis energy. The jet-cooled absorption spectrum near the channel (1) threshold [D0(H+NCO)] was obtained using two-photon excitation via the S1 state, terminating in a fluorescent product. The absorption spectrum is similar to the NCO yield spectrum, and its intensity does not diminish noticeably above D0(H+NCO), indicating that dissociation near threshold is slow. The NCO product near threshold is cold, as is typical of a barrierless reaction. NH (a1Δ) products appear first at 42 840 cm−1, but their yield is initially very small, as evidenced also by the insignificant decrease in the NCO yield in the threshold region of channel (2). The NH (a1Δ) yield increases faster at higher photolysis energies and the linewidths increase as well. At the channel (2) threshold, the NH (a1Δ) product is generated only in the lowest rotational level, J=2, and rotational excitation increases with photolysis energy. We propose that in the range 260–230 nm, HNCO (S1) undergoes radiationless decay terminating in S0/T1 followed by unimolecular reaction. Decompositions via channels (1) and (2) proceed without significant exit channel barriers. At wavelengths shorter than 230 nm, the participation of an additional, direct pathway cannot be ruled out. The jet-cooled photofragment yield spectra allow the determination, with good accuracy, of thermochemical values relevant to HNCO decomposition. The following heats of formation are recommended: ΔH0f(HNCO)=−27.8±0.4 kcal/mol, and ΔH0f(NCO)=30.3±0.4 kcal/mol. These results are in excellent agreement with recent determinations using different experimental techniques. © 1996 American Institute of Physics.

Type of Medium: Electronic Resource

URL: http://dx.doi.org/10.1063/1.472665

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Fragment recoil anisotropies in the photoinitiated decomposition of HNCO (1999)

Zyrianov, M. ; Droz-Georget, Th. ; Reisler, H.

College Park, Md. : American Institute of Physics (AIP)

The Journal of Chemical Physics 110 (1999), S. 2059-2068

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ISSN: 1089-7690

Source: AIP Digital Archive

Topics: Physics , Chemistry and Pharmacology

Notes: The photofragment ion imaging technique is used to determine product recoil anisotropy parameters, β, and correlated state distributions in the S1(1A″)←S0(1A′) photoinitiated decomposition of HNCO into three competing channels: (1) 3NH+CO, (2) H+NCO, and (3) 1NH+CO [where 3NH and 1NH denote NH(X 3Σ−) and NH(a 1Δ), respectively]. In particular, the region in the vicinity of the 1NH+CO threshold is investigated. The measured recoil anisotropies fall into two distinct groups corresponding to time scales of 〈1 ps (β〈−0.6), and 〉5–10 ps (β≅0.0). With 230.1 nm photolysis, CO(J=0–14) originating in channel (3) is produced with β=−0.8±0.05 via direct dissociation on S1 above a barrier of 470±60 cm−1. CO at low J-states appears with most of the available energy in the translational degree of freedom and is correlated with 1NH in its lowest rotational states. A small contribution to channel (3) from S0 dissociation (observed mainly for J=14,15) gives rise to an isotropic recoil distribution, and a hotter correlated 1NH rotational distribution. At the same wavelength, CO correlated with 3NH is identified by its high translational energy and exhibits an isotropic angular distribution. We propose that the pathway leading to its formation is S1→S0→T1. H-atom signals from channel (2) have isotropic angular distributions at photolysis wavelengths 243−215 nm; this places a lower limit of 8140 cm−1 on the barrier to direct dissociation on S1 to channel (2). The 〉5 ps time scale for the appearance of channel (2) implies dissociation on S0 following internal conversion. The mechanism described here for the one-photon decomposition of HNCO in the wavelength region 260-230 nm is in accord with other available experimental and theoretical findings. © 1999 American Institute of Physics.

Type of Medium: Electronic Resource

URL: http://dx.doi.org/10.1063/1.477874

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Photoinitiated decomposition of HNCO near the H+NCO threshold: Centrifugal barriers and channel competition (1999)

Zyrianov, M. ; Sanov, A. ; Droz-Georget, Th. ; [et al.]

College Park, Md. : American Institute of Physics (AIP)

The Journal of Chemical Physics 110 (1999), S. 10774-10783

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ISSN: 1089-7690

Source: AIP Digital Archive

Topics: Physics , Chemistry and Pharmacology

Notes: The decomposition of jet-cooled HNCO is investigated near the H+NCO channel threshold [D0(H+NCO)=38 370 cm−1]. Dissociation to H+NCO at energies 17–411 cm−1 above D0(H+NCO) proceeds on the ground potential energy surface (S0), apparently without a barrier. The rotational state distributions of the NCO(X 2Π3/2,0010) fragment are well described by phase space theory (PST), provided that dynamical constraints are included. These constraints are associated with long range (4–7 Å) centrifugal barriers, which are significant even near threshold because of the small reduced mass of H+NCO, and result in a fraction of energy deposited in fragment rotation much smaller than predicted by unconstrained PST. The influence of orientation averaging on the attractive, long-range part of the potential is discussed, and it is argued that angular averaging with respect to the center of mass of the rotating polyatomic fragment results in a shift in the effective potential origin, accompanied by an attenuation of the magnitude of the potential compared to its value for fixed H–N distance. Following initial S1(1A″)←S0(1A′) excitation and internal conversion to S0, HNCO(S0) decays both via unimolecular decomposition of H+NCO and intersystem crossing to the dissociative first triplet state, T1 [yielding NH(X 3Σ−)+CO products]. The competition between the two processes is interrogated by monitoring changes in the relative yields of NCO and NH(X 3Σ−) as a function of excitation energy. It is concluded that near D0(H+NCO), the S0→T1 intersystem crossing rate is several-fold faster than the H+NCO unimolecular decomposition rate. © 1999 American Institute of Physics.

Type of Medium: Electronic Resource

URL: http://dx.doi.org/10.1063/1.478998

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