Supporting Information. and NaCl at the Air/Aqueous Interface revealed by Heterodyne Detected. Phase-Sensitive Sum Frequency

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1 Suppoting Infomation lectic Field Revesal of Na SO 4, (NH 4 ) SO 4, and Na CO 3 elative to CaCl and NaCl at the Ai/Aqueous Inteface evealed by Heteodyne Detected Phase-Sensitive Sum Fequency Wei Hua, Aaon M. Jubb, and Heathe C. Allen* Depatment of Chemisty, The Ohio State Univesity, 100 West 18th Ave, Columbus, OH, 4310 allen@chemisty.ohio-state.edu Re χ () specta The Re χ () specta of wate molecules at vapo/aqueous solution intefaces ae shown in Figue S1. The esult of spectal convolution of the Im χ () component with the Re χ () component accounts fo the changes of spectal line shape elative to the spectum obseved with conventional VSFG (Fig. 1a). In the case of the calcium chloide salt solution VSFG spectum (Fig. 1a), this convolution explains the enhancement of the 3300 cm -1 featue, wheeas fo the othe salt solutions convolution with the Re χ () component does not poduce significant spectal change. S1

2 M Na CO 3 1 M Na SO 4 Reχ () (a.u.) M (NH 4 ) SO 4 M CaCl M NaCl Incident Infaed (cm -1 ) Figue S1. PS-SFG Re χ () specta of wate molecules at vapo/aqueous solution intefaces of, 1.8 M CaCl, 1.8 M NaCl, 1.1 M Na CO 3, 1.1 M Na SO 4, and 1.1 M (NH 4 ) SO 4 salt solutions. xpeimental Methods and Data Pocessing Pocedues A titanium:sapphie oscillato (79 nm) double egeneative amplifie system (1 khz) (Specta-Physics) was used fo the visible and infaed light geneation. The amplifies geneate a visible beam of ps pulses at 79 nm (17 cm -1 bandwidth, > 500 mw), while ~ 85 fs pulses at 79 nm ( nm bandwidth, > 1 W) ae used to geneate a tunable boadband infaed beam in an optical paametic amplifie (Light Convesion, TOPAS). This is followed by a nonlinea diffeence-fequency geneation system (Light Convesion, NDFG connected to the TOPAS). The full spectal bandwidth of the geneated boadband infaed beam is ~500 cm -1 in the egion unde investigation and the aveage powe of the visible beam and the infaed beam wee 300 µj and 10 µj, espectively. The visible beam, s-polaized and 79 nm, and the infaed beam, p-polaized, wee spatially and tempoally ovelapped on the stage (fo s and z-cut quatz) with incident angles of ~50 and ~60, espectively, to geneate the sum fequency beam, which was s-polaized. S

3 The PS-SFG and detection setup used hee is simila to the system epoted by Tahaa and co-wokes, 1 which is based on heteodyne detection of boad band intensities and Fouie tansfom analysis. The impotant pat of the optical configuation in ou SFG system was edesigned fo the PS-SFG application as is illustated in Figue S.,3 visible, s-polaized silica plate gold concave IR, p-polaized /quatz GaAs filtes and polaize Monochomato + CCD Figue S. Schematic of the heteodyne-detected PS-SFG optical configuation. Filtes and polaize denote two shot-pass filtes, a notch filte, and a Glan-Thompson polaize espectively fom left to ight. Afte eflection fom the /efeence, the incident visible, infaed, and geneated sum fequency (s-polaized) beams wee efocused by a gold concave mio (f = 100 mm) onto a GaAs (Lambda Pecision Optics) suface to geneate anothe sum fequency beam (local oscillato, ). The two sum fequency beams fom diffeent stages geneated an intefeence finge in the fequency domain by passing the sum fequency beam geneated fom the (o quatz efeence) though a 1 mm thick silica plate positioned befoe the gold concave mio, esulting in a time delay of ~.6 ps. The intefeogam was stetched in a monochomato (Acton Reseach, SpectaPo SP-500 monochomato with a 100 g/mm gating blazed at 750 nm) and detected by a liquid-nitogen cooled chage-coupled device (CCD) (Rope Scientific, pixel aay, Spec-10:400B: LN400B back illuminated CCD). Special attention was given to the shape of image on the CCD aay. If the image was slightly tilted, data points wee consistently shifted elative to the cental pixel line on the image to avoid integated signal attenuation. The height of the suface was checked by the image on the CCD. Retaining S3

4 the same height is citical fo accuate phase detemination. Duing the expeiments, the height of the efeence suface (quatz) and the suface wee identical, within ou measuement ability. Neat wate was used as a efeence fo the pixel height. Height accuacy was bette than 3 µm, whee each CCD pixel is 0 x 0 µm. The total detected intensity can be descibed as: 1 HD SFG tot I = + * * = + + exp( iω t) + exp( iω t) denotes the sum fequency beam fom s o quatz and denotes the sum fequency beam fom GaAs. t is the ~.6 ps time diffeence between two sum fequency beams. The aw intefeogams (Figue S3) wee invese Fouie tansfomed to the time domain using OiginPo softwae (vesion 7.5). The and signals at t = 0 in time domain wee filteed out and only the coss tems wee kept and followed by Fouie tansfom back to the fequency domain. The esulting fequency specta contain phase infomation (φ) of the complex * exp(iω t) (Figue S3). The quatz spectum was used as a efeence, because quatz did not have any appaent esonance in this infaed egion, and the phase of quatz was egaded as a constant. Theefoe, the eal and imaginay χ () specta can be obtained by dividing the intefeogam by the quatz efeence spectum though which the contibution fom is completely emoved. The final specta wee nomalized to the eflectivities of the incident visible and IR beams on quatz and on the s. The expession of the Im χ () spectum is as follows: Im χ () vis, quatz vis, IR, quatz IR, quatz sin( ϕ ϕ quatz ) is the eflectivity of the incident visible o IR beams on quatz o the. The phases of the quatz and (φ) wee obtained diectly fom the Fouie tansfomation. Final specta wee aveaged ove the two consecutive uns with 5 min integation times fo each. The measued epoducibility of the intensity when the was changed was within 5%. S4

5 50000 Counts (a.u.) Amplitude (a.u.) Incident infaed (cm -1 ) Time (ps) 10 Counts (a.u.) Incident infaed (cm -1 ) Figue S3. Uppe panel: Raw intefeogam of z-cut quatz with GaAs. Middle panel: Time domain eal (black) and imaginay (ed) signals. The coss tem at ~.6 ps is extacted to yield the heteodyne fequency specta. Lowe panel: Real (black) and imaginay (ed) pats of heteodyne fequency specta of z-cut quatz with GaAs. Phase infomation (φ) is theefoe obtained. The heteodyne-detected PS-SFG specta ae shown in the main manuscipt and hee fom 3000 to 3500 cm -1. Ou cuent IR pofile has intensity that is lowe nea 3100 cm -1 as compaed to 3400 cm -1 (Figue S3, uppe panel), which can slightly distot the nomalized intensity nea 3100 cm -1. Wok is unde way in ou laboatoy to futhe expand ou available S5

6 PS-SFG bandwidth. To discuss the delicate spectal changes in the ai/aqueous inteface the phase stability and accuacy need to be caied out with exteme cae. As mentioned above, when doing data pocessing, the Im χ () of the is calculated fom nomalizing the phase of (φ ) by the phase of quatz efeence (φ quatz ). Howeve, duing the expeimental opeation, the way the quatz efeence is mounted on the stage was impefect, so the phase (angle) of z-cut quatz might vay on diffeent days. This may esult in an uncetainty with assigning the absolute phase of ou quatz efeence; wok is undeway in ou laboatoy to impove this. Theefoe, a phase dift of the quatz efeence could be obseved occasionally and hence a phase coection pocedue was applied with espect to ou peviously obtained aveaged quatz phase data to eliminate (o compensate) fo this uncetainty. Ou spectal esolution (and epoducibility) ae less than optimal and ou wate Im χ () spectum, hence, has a 0 ±5 phase esolution (uncetainty). This is the main eason fo the shift of the cossing point in the published Im χ () spectum fo,3 and this manuscipt. Citical hee is that all Im χ () specta of salt solutions ae compaed to the Im χ () spectum. Thus ou intepetation is mainly based on the elative diffeence fom to that of the salt solution systems. Duing daily opeation, the quatz efeence was neve moved when measuing the specta fo and all the salt solution s, and the wate was e-measued aound evey thee hous to ensue the stability and epoducibility of the phase of the wate Im χ () spectum obtained in the same day. This ensued eliable phase epoducibility between measued and salt solutions Im χ () specta. Hee, eplicate Im χ () specta of, 1.1 M Na CO 3, 1.8 M CaCl and 1.8 M NaCl salt solutions measued on diffeent days ae shown in Figue S4. Although fine spectal line shape changes ae obseved, the geneal shape of Im χ () specta on diffeent days fo the same agee within expeimental eo. The elative diffeence obseved in the Im χ () specta wee epoduced on diffeent days. S6

7 Imχ () (a.u.) Imχ () (a.u.) M Na CO M Na CO Incident Infaed (cm -1 ) Imχ () (a.u.) Imχ () (a.u.) M CaCl M NaCl M CaCl M NaCl Incident Infaed (cm -1 ) Figue S4: Repoducibility testing: Im χ () specta of, 1.1 M Na CO 3, 1.8 M CaCl and NaCl salt solutions measued on diffeent days. Refeences (1) Nihonyanagi, S.; Yamaguchi, S.; Tahaa, T. Diect vidence fo Oientational Flip-Flop of Wate Molecules at Chaged Intefaces: A Heteodyne-Detected Vibational Sum Fequency Geneation Study. J. Chem. Phys. 009, 130, () Chen, X. K.; Hua, W.; Huang, Z. S.; Allen, H. C. Intefacial Wate Stuctue Associated with Phospholipid Membanes Studied by Phase-Sensitive Vibational Sum Fequency Geneation Spectoscopy. J. Am. Chem. Soc. 010, 13, (3) Hua, W.; Chen, X. K.; Allen, H. C. Phase-Sensitive Sum Fequency Revealing Accommodation of Bicabonate Ions, and Chage Sepaation of Sodium and Cabonate Ions within the Ai/Wate Inteface. J. Phys. Chem. A 011, 115, S7