J/A+A/575/A121 Photodesorption of H2O, HDO, and D2O (Arasa, C. +, 2015) ================================================================================ Photodesorption of H2O, HDO, and D2O ice and its impact on fractionation Arasa, C., Koning, J., Kroes, G,-J., Walsh, C., & van Dishoeck, E. F. =2015A&A...575A.121A ================================================================================ Keywords: astrochemistry - molecular processes - ISM: molecules - solid state:volatile Abstract: The HDO/H2O ratio measured in interstellar gas is often used to draw conclusions on the formation and evolution of water in star-forming regions and, by comparison with cometary data, on the origin of water in Earth. In cold cores and in the outer regions of protoplanetary disks, the primary source of gas-phase water comes from photodesorption of water ice. This paper presents photodesorption efficiencies for all water ice isotopologues, together with fitting formulae for implementation in astrochemical models. The results are used to investigate to what extent the gas-phase HDO/H2O ratio reflects that present in the ice or whether fractionation can occur during the photodesorption process. Classical molecular dynamics simulations have been performed to study the photodesorption of X (X=H,D) atoms, OX radicals, and X2O and HDO molecules following photodissociation of H2O, D2O, and HDO in H2O amorphous ice at ice temperatures from 10 - 100 K. Significant isotope effects are found for all possible products: (1) H atom photodesorption probabilities from H2O ice are larger than those for D atom photodesorption from D2O ice by a factor of 1.1; the ratio of H and D photodesorbed upon HDO photodissociation is a factor of 2. This process will enrich the ice in deuterium atoms over time; (2) the OD/OH photodesorption ratio upon D2O and H2O photodissociation is on average a factor of 2, but the OD/OH photodesorption ratio upon HDO photodissociation is almost constant at unity for all ice temperatures; (3) the total HDO/H2O ratio in the gas is equal to the HDO/H2O ratio in the ice, therefore, there is no isotope fractionation when HDO and H2O photodesorb from the ice. However, the enrichment of the ice in D atoms due to photodesorption can over time lead to an enhanced HDO/H2O ratio in the ice, and, when photodesorbed, also in the gas. The extent to which the ortho/para ratio of H2O can be modified by the photodesorption process is discussed briefly as well. Description: The data listed in this electronic table are the results of molecular dynamics simulations of water ice photodissociation and subsequent desorption mechanisms. Upon photoexcitation and dissociation of a water molecule within an ice mantle frozen onto an interstellar or circumstellar dust grain, there are several potential chemical outcomes, the probabilities for which are dependent upon ice temperature and ice monolayer. Here, we list the probabilities per monolayer as a function of temperature for each outcome following the dissociation of a H2O, HDO, or D2O molecule in water (H2O) ice. DOH refers to the dissociation of HDO into D + OH and HOD to the dissocation of HDO into H + OD; hence, the data contain probabilies for 4 dissociation events. These data have been compiled from the raw simulation data which considers around 6000 trajectories or initial conditions. File Summary: -------------------------------------------------------------------------------- FileName Lrecl Records Explanations -------------------------------------------------------------------------------- ReadMe 80 ... This file outcomes.dat 96 70 Probabilities as function of monolayer and ice temperature for each possible outcome following the dissociation of a H2O, HDO, or D2O molecule in water (H2O) ice. table2.dat 35 100 Probabilities and associated errors as a function of monolayer and ice temperature for H or D atom desorption following the dissociation of a H2O, HDO,or D2O molecule in water (H2O) ice. table4.dat 36 100 Probabilities and associated errors as a function of monolayer and ice temperature for OH or OD desorption following the dissociation of a H2O, HDO,or D2O molecule in water (H2O) ice. table5.dat 52 100 Probabilities and associated errors as a function of monolayer and ice temperature for XYO (X,Y=H,D) desorption and H2O desorption via kick out following the dissociation of a XYO molecule in water (H2O) ice. -------------------------------------------------------------------------------- Byte-by-byte description of file: outcomes.dat -------------------------------------------------------------------------------- Bytes Format Units Label Explanations -------------------------------------------------------------------------------- 1- 3 A3 --- Spec Species names 6- 9 F4.1 K Tice Ice temperature 12 I1 --- ML Monolayer number 15- 24 E10.4 --- P1 Probability for outcome 1 (see Note 1) 27- 36 E10.4 --- P2 Probability for outcome 2 (see Note 1) 39- 48 E10.4 --- P3 Probability for outcome 3 (see Note 1) 51- 60 E10.4 --- P4 Probability for outcome 4 (see Note 1) 63- 72 E10.4 --- P5 Probability for outcome 5 (see Note 1) 75- 84 E10.4 --- P6 Probability for outcome 6 (see Note 1) 87- 96 E10.4 --- KO Probability for kick out (see Note 1) -------------------------------------------------------------------------------- Note (1): Outcome 1: Xdes + OYtrap i.e. H/D desorbs and OH/OD is trapped Outcome 2: Xtrap + OYdes i.e. H/D is trapped and OH/OD desorbs Outcome 3: Xdes + OYdes i.e. both products desorb Outcome 4: XYOdes i.e. H/D and OH/OD recombine and desorb (direct mechanism) Outcome 5: Xtrap + OYtrap i.e. both products trapped Outcome 6: XYOtrap i.e. H/D and OH/OD recombine and remain trapped Kick out : H2Okick i.e. H/D kicks out a neighbouring H2O molecule (indirect mechanism) -------------------------------------------------------------------------------- Byte-by-byte description of file: table2.dat -------------------------------------------------------------------------------- Bytes Format Units Label Explanations -------------------------------------------------------------------------------- 1- 4 F4.1 K Tice Ice temperature 7- 9 A3 --- Sexc Photoexcited species (XYO where X,Y=H,D) 12 A1 --- Sdes Desorbed species (H or D) 15 A1 --- ML Monolayer number or average (see Note 1) 18- 25 E8.2 --- Pdes Probability for X atom desorption following XYO dissociation 28- 35 E8.2 --- Edes Associated error for desorption probability -------------------------------------------------------------------------------- Note (1): The average values over the top four monolayers for each ice temperature and for each probability are also listed and indicated with an "A" in the second column. -------------------------------------------------------------------------------- Byte-by-byte description of file: table4.dat -------------------------------------------------------------------------------- Bytes Format Units Label Explanations -------------------------------------------------------------------------------- 1- 4 F4.1 K Tice Ice temperature 7- 9 A3 --- Sexc Photoexcited species (XYO where X,Y=H,D) 12- 13 A2 --- Sdes Desorbed species (OH or OD) 16 A1 --- ML Monolayer number or average (see Note 1) 19- 26 E8.2 --- Pdes Probability for OX desorption following XYO dissociation 29- 36 E8.2 --- Edes Associated error for desorption probability -------------------------------------------------------------------------------- Note (1): The average values over the top four monolayers for each ice temperature and for each probability are also listed and indicated with an "A" in the second column. -------------------------------------------------------------------------------- Byte-by-byte description of file: table5.dat -------------------------------------------------------------------------------- Bytes Format Units Label Explanations -------------------------------------------------------------------------------- 1- 4 F4.1 K Tice Ice temperature 7- 9 A3 --- Sexc Photoexcited species (XYO where X,Y=H,D) 12 A1 --- ML Monolayer number or average (see Note 1) 15- 22 E8.2 --- Pdes Probability for XYO desorption following XYO dissociation 25- 32 E8.2 --- Edes Associated error for desorption probability 35- 42 E8.2 --- Pkick Probability for H2O desorption via the kick out mechanism following XYO dissociation 45- 52 E8.2 --- Ekick Associated error for kickout probability -------------------------------------------------------------------------------- Note (1): The average values over the top four monolayers for each ice temperature and for each probability are also listed and indicated with an "A" in the second column. -------------------------------------------------------------------------------- References: 2006JChPh.124f4715A 2008A&A...491..907A 2010JChPh.132r4510A 2011JChPh.134p4503A 2013JChPh.138j4701K ================================================================================ (End) Catherine Walsh [Leiden Observatory, The Netherlands] 25-Mar-2015