Near ultraviolet photolysis of phosphine studied by H atom photofragment translational spectroscopy

Abstract

. Can. J. Chem. 72,977 (1 994).We report time-of-flight measurements of the H atom fragments arising in the photodissociation of phosphine at three near ultraviolet excitation wavele_ngths -2 18.2 nm, 221.6 nm, and 228.0 nm. For these wavelengths, energetic considerations dictate that ground state PH2(X) fragments must be the partner product of the primary photolysis event. The measured H atom kinetic energy distributions indicate that internal excitation of these partner fragments accounts for the major part (362%) of the available energy, in excess of that required to break the H-pH2 bond, provided by the photon. Inspection of the poorly resolved structure evident near the peak of the H atom kinetic energy distribution suggests the presence of substantial bending excitation and a-axis rotational excitation in the pH2 partner fragment. The_H atom kinetic energy distribution also shows clear evidence of atoms arising from secondary photolysis of the primary PH,(X) fragments. This second_ary photo-dissociation is observed to show a marked anisotropy, consistent with a perpendicular photo-excitation of the PH2(X) fragment. Arguments are presented which indicate that the secondary photolysis leads both to the formation of PH(X) fragments, carrying high levels of rotational excitation, and to the three-body atomisation process. On a effectuC des mesures en temps de vol des atomes d'hydrogkne, sous la forme de fragments provenant de la photodissociation de la phosphine B trois longueurs d'onde d'excitation -218,2, 221,6 et 2230 nm -proches de l'ultraviolet. Pour ces longueurs d'onde, des considCrations CnergCtiques dictent que les fragments PH2(X) de 1'Ctat fondamental doivent Etre les produits partenaires de la photolyse primaire. Les distributions de l'knergie cinCtique mesurCes pour les atomes d'hydrogbne indiquent que l'excitation interne de ces fragments partenaires tient compte de la plus grande partie (362%) de 1'Cnergie disponible B partir du photon, ce qui est supCrieur B 1'Cnergie requise pour briser la liaison H-pH2. Un examen minutieux de la structure ma1 rCsolue prCsente prks du pic de la distribution de 1'Cnergie cinCtique des atomes d'hydrogkne suggkre la prCsence d'une excitation importante de dkformation et d'une excitation rotationnelle le long de l'axe a du fragment partenaire pH2. La distribution de 1'Cnergie cinCtique des_atomes d'hydrogkne montre des preuves de la prCsence d'atomes provenant d'une photoIyse secondaire des fragments PH2(X) primaires. L'observation de cette photodissoc~ation secondaire dCmontre la prCsence d'une anisotropie marquCe qui est en accord avec une photoexcitation du fragment PH2(X). On prCsente des arguments indiquant que la photolyse secondaire conduit B la formation de fragments PH(X) portant des niveaux ClevCs d'excitation rotationnelle ainsi qu'au processus d'atomisation B trois corps.[Traduit par la redaction]

AsH3 Ultraviolet Photochemistry

“…The distribution shown in Figure 6 indicates that reaction 3 dominates, with high AsH 2 (X ˜) rovibrational excitation. This is reasonable in light of the photodissociation dynamics of PH 3 [21][22][23] and NH 3 . 12-14,17 Ultraviolet photoexcitation results in a change of equilibrium geometry.…”

“…For example, Lambert et al observed PH 2 with substantial bending excitation and a-axis rotation following the ultraviolet photolysis of PH 3 . 21 In contrast, it is known that NH 2 is formed with a relatively modest amount of bending excitation. [12][13][14]17 The equilibrium bond angles for AsH 3 (A ˜) and AsH 2 (X ˜) are 112°(an estimate based on AsH 3 + and PH 3 (A ˜)) and 90.4°, 39 respectively.…”

“…It has been suggested that the PH 3 Ã/X̃ conical intersection affects the dissociation dynamics in a manner that is analogous to the case of NH 3 . Several experimental studies have shown that PH 2 is born with substantial internal excitation, − although the exact nature of this excitation is more difficult to discern than for NH 2 . Lambert et al investigated the UV photolysis of PH 3 by using high- n Rydberg time-of-flight (HRTOF) spectroscopy.…”

“…Several experimental studies have shown that PH 2 is born with substantial internal excitation, − although the exact nature of this excitation is more difficult to discern than for NH 2 . Lambert et al investigated the UV photolysis of PH 3 by using high- n Rydberg time-of-flight (HRTOF) spectroscopy. They found that PH 2 (X̃) rovibrational excitation accounts for, on average, ∼62% of the available energy.…”

πσ* excited states in molecular photochemistry

“…79 The A ˜state of PH 3 has a non-planar equilibrium geometry but, as in ammonia, the A ˜and X ˜state PESs exhibit a CI at planar configurations and extended R H-PH 2 , 79 thereby allowing efficient dissociation to ground state H + PH 2 (X ˜) products. 17,80 The limited available data relating to AsH 3 photochemistry (at 193.3 nm) suggests that this molecule also exhibits broadly similar dissociation dynamics. 81 Many of the parallels between H 2 O and CH 3 OH are evident also when comparing NH 3 and CH 3 NH 2 .…”