Supporting Information

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1 Supportng Informaton Water structure at the ar-aqueous nterface of dvalent caton and ntrate solutons Man Xu, Rck Spnney, Heather C. Allen* Fresnel factors and spectra normalzaton The ntensty, I, s proportonal to the absolute square of the ectve sum frequency susceptblty,, and to the ntenstes of the ble and nfrared ncdent pulses by, I 3 8π ω sec θ = ( ( 3 I ω I ω ( c n n n wth [ eˆ ( ω ] :[ ( ω eˆ( ω ] [ ( ω eˆ( ω ] = where n (ω s the refractve ndex of the bulk medum at frequency ω, eˆ s the unt electrc feld vector, and (ω s the Fresnel factor at frequency ω., the ectve sum frequency susceptblty, s polarzaton dependent. Under polarzaton combnatons ssp, sps, pss and ppp, can be expressed as equaton 3, 4, 5, 6, respectvely.,

2 , ssp = z (3, sps = yzy (4, pss = z (5, ppp + + = z xzx z z (6 Accordng to above equatons, the ntensty s assocated wth the Fresnel factors. Therefore, when performng the normalzaton of spectra, the Fresnel factors should be consdered n addton to nfrared and ble ntenstes. The Fresnel factors are functons of the refractve ndces of the beam n dfferent meda, and the expermental geometry, n cosγ n cosγ + n = (7 n n + n cosγ = (8 n n ω (9 ( = ( cos + ( cos '( n ω γ n ω θ n ω where n m (ω s the refractve ndex of the bulk medum m (m =,, at frequency ω ( =,,, n (ω s the ectve refractve ndex of the nterface that can be estmated by equaton (0 shown below. γ s refractve angle nto medum defned by

3 Snell s law n = n( ω snγ. θ s the ncdent or reflecton angle from the nterface normal for the beam n consderaton. '( n ω = n 4n ( n (0 For the ssp-polarzed spectra at the ar-aqueous nterface, the relevant Fresnel factors are: = ( n n + n cosγ n n + n = ( cosγ n n ω (3 ( = ( cos + ( cos '( n ω γ n ω θ n ω Refractve ndex of aqueous phase n, reflecton angle θ of, refractve angles γ of and, and refractve ndex of the nterface n are all frequency dependent and therefore are consdered when calculatng the Fresnel factors and performng the spectra normalzaton. Refractve ndces of water at dfferent wavelengths or wavenumbers were obtaned from lterature 3,4 and are plotted n Fgure S. For the system used n ths work, the nput ble s at 53 nm, and vares from 800 cm - to 3800 cm -. The ncdent angles of ble and beams were set to be 45 and 53 to the surface normal. Fgure S gves the calculated Fresnel factors (ω and (ω at the ar-neat water nterface. (ω of neat water has a value of

4 Fgure S. (a Refractve ndex of water as a functon of wavelength n the ble regon; (b Refractve ndex of water as a functon of wavenumbers of. 3,4 Fgure S. Fresnel factors (ω and (ω of neat water as a functon of wavenumber of ncdent. 4

5 Fgure S3a shows the spectra of neat water and ntrate aqueous solutons normalzed to the real-tme profles. In the spectra shown n Fgure S3b-c, ( sec θ ω as well as the ntenstes were employed for the spectra normalzaton. In the spectra shown n Fgure S3b, we assumed that the refractve ndces of neat water and ntrate aqueous solutons are the same. Actually accordng to our RAS and Kramers-Krong results, the refractve ndex dfferences between neat water and the ntrate solutons used n ths study are wthn 5-0%. A dfference of 6% n refractve ndex s therefore utlzed n the normalzaton shown n Fgure S3c. Small dfferences n the hydrogen-bonded OH stretchng regon are observed when usng dfferent normalzaton methods. In the spectra shown n Fgure S3b-c that are normalzed to the Fresnel factors and the real-tme, the ects of dfferent refractve ndces n dfferent meda and the ects of expermental geometry that vares from laboratory to laboratory have been consdered. However, the same expermental geometry was utlzed for the experments of water and ntrate solutons, and smlar spectral changes are observed n the spectra shown n Fgure S3b-c compared to the spectra n Fgure S3a. Therefore t s reasonable that the artcle dscusson focuses on the spectra normalzed to only. 5

6 Fgure S3. ssp-polarzed spectra at the ar aqueous nterface: (a spectra normalzed to real-tme ; (b spectra normalzed to F factor sec ( ( ( (F= ω θ ω ω and real-tme, assumng that neat water and ntrate solutons have the same refractve ndces; (c spectra normalzed to F factor (F= ω sec θ ( and real-tme, assumng that the refractve ndces of ntrate solutons are 6% hgher than that of neat water. 6

7 In Fgure S4, Raman spectra of NaCl, NaBr, and NaNO 3 aqueous solutons are shown for comparson. Note that the bromde and chlorde solutons enhance the 3400 cm - regon over that of the ntrate system. Fgure S4. Raman spectra of NaCl, NaBr, and NaNO 3 aqueous solutons. Spectrum of neat water s plotted for comparson. Supportng Informaton References ( Zhuang, X.; Mranda, P. B.; Km, D.; Shen, Y. R. Phys. Rev. B 999, 59, Gan, W.; Wu, D.; Zhang, Z.; Guo, Y.; Wang, H.-F. Chnese Journal of Chemcal Physcs 006, 9, 0-4. (3 Querry, M. R.; Welczka, D. M.; Segelsten, D. J. Water (H O. In Handbook of Optcal Constants of Solds II; ed.; Palk, E. D., Eds.; Academc: Boston, 99; Vol.; pp (4 Segelsten, D. J. The Complex Refractve Index of Water. Unversty of Mssour