Data-[{0.720093, 0.295084}, {0.717593, 0.294154}, {0.71511,
  0.293408}, {0.712644, 0.292561}, {0.710195, 0.291841}, {0.707763,
  0.290859}, {0.705347, 0.289796}, {0.702948, 0.288597}, {0.700565,
  0.287483}, {0.698198, 0.286434}, {0.695847, 0.285242}, {0.693512,
  0.28415}, {0.691193, 0.283081}, {0.688889, 0.281928}, {0.6866,
  0.28078}, {0.684327, 0.279592}, {0.682068, 0.278541}, {0.679825,
  0.277436}, {0.677596, 0.27651}, {0.675381, 0.275654}, {0.673181,
  0.274795}, {0.670996, 0.273949}, {0.668824, 0.272901}, {0.666667,
  0.271992}, {0.664523, 0.271335}, {0.662393, 0.270664}, {0.660277,
  0.269989}, {0.658174, 0.26921}, {0.656085, 0.268645}, {0.654008,
  0.26808}, {0.651945, 0.267374}, {0.649895, 0.266843}, {0.647858,
  0.266955}, {0.645833, 0.267217}, {0.643821, 0.267661}, {0.641822,
  0.267598}, {0.639835, 0.267401}, {0.63786, 0.267703}, {0.635897,
  0.268113}, {0.633947, 0.269681}, {0.632008, 0.27114}, {0.630081,
  0.272627}, {0.628166, 0.274489}, {0.626263, 0.276107}, {0.624371,
  0.277585}, {0.62249, 0.278766}, {0.620621, 0.280244}, {0.618762,
  0.281839}, {0.616915, 0.283631}, {0.615079, 0.284915}, {0.613254,
  0.285762}, {0.61144, 0.286192}, {0.609636, 0.288746}, {0.607843,
  0.29093}, {0.606061, 0.292403}, {0.604288, 0.293922}, {0.602527,
  0.297144}, {0.600775, 0.299701}, {0.599034, 0.302432}, {0.597303,
  0.304511}, {0.595581, 0.309842}, {0.59387, 0.313888}, {0.592168,
  0.318189}, {0.590476, 0.319657}, {0.588794, 0.320562}, {0.587121,
  0.321366}, {0.585458, 0.321272}, {0.583804, 0.31859}, {0.58216,
  0.315947}, {0.580524, 0.313623}, {0.578898, 0.310534}, {0.577281,
  0.304952}, {0.575673, 0.298553}, {0.574074, 0.290369}, {0.572484,
  0.279767}, {0.570902, 0.26778}, {0.56933, 0.2571}, {0.567766,
  0.244668}, {0.56621, 0.22961}, {0.564663, 0.215466}, {0.563124,
  0.200814}, {0.561594, 0.184229}, {0.560072, 0.169114}, {0.558559,
  0.153148}, {0.557053, 0.137587}, {0.555556, 0.122651}, {0.548187,
  0.108596}, {0.546737, 0.092986}, {0.545295, 0.0785259}, {0.54386,
  0.0709461}, {0.542432, 0.0609015}, {0.541012, 0.0524228}, {0.5396,
  0.0465767}, {0.538194, 0.0402518}, {0.536797, 0.0339568}, {0.535406,
   0.0290519}, {0.534022, 0.0210021}, {0.532646,
  0.0163296}, {0.531277, 0.0125908}, {0.529915, 0.011545}, {0.528559,
  0.00787535}, {0.527211, 0.00539227}, {0.525869,
  0.00218363}, {0.524535, -4.67229*10^-6}}]


    data = Import["E:\FITTING DATA\Sheet1.txt", "Table"];
        original =
         ListLinePlot[data, AxesLabel -> {count, intensity},
          PlotStyle -> {Red}, Frame -> True]
        f1 = a*UnitStep[x - b]*c*
          Sqrt[x - b]*(Pi*Sqrt[d]/Sqrt[x - b]*Exp[Pi*Sqrt[d]/Sqrt[x - b]])/
           Sinh[Pi*Sqrt[d]/Sqrt[x - b]]
        f2 = a*d*Sum[(4*Pi)/n^3*DiracDelta[x - b + d/n^2], {n, 1, 1}]
        f3 = Convolve[
          a*d*Sum[(4*Pi)/n^3*DiracDelta[x - b + d/n^2], {n, 1, 1}],
          Re[Erfc[([Delta] + I*x)/(Sqrt[2]*[Sigma])]/
            E^((x - I*[Delta])^2/(2*[Sigma]^2))], x, y]
        fit2 = NonlinearModelFit[
           data, {a*UnitStep[y - b]*c*
              Sqrt[y -
                b]*(Pi*Sqrt[d]/Sqrt[y - b]*Exp[Pi*Sqrt[d]/Sqrt[y - b]])/
               Sinh[Pi*Sqrt[d]/Sqrt[y - b]] +

             Convolve[
              a*d*Sum[(4*Pi)/n^3*DiracDelta[x - b + d/n^2], {n, 1, 1}],
              Re[Erfc[([Delta] + I*x)/(Sqrt[2]*[Sigma])]/
                E^((x - I*[Delta])^2/(2*[Sigma]^2))], x, y],
            b < Min[data[[All, 1]]],
                 Inequality[10, Less, a, Greater, 15], 0.01 < c < 0.1,
            Inequality[0.001, Less, d, Greater, 0.01]}, {{a, 10}, {b, -1}, c,
            d, [Delta], [Sigma]}, y];
        fit2["BestFitParameters"]
        Show[ListPlot[data, PlotStyle -> {Red},
          AxesLabel -> {ev, absorbstion}],
         Plot[fit2[y], {y, 1.3, 0.55}, PlotStyle -> {Blue}], Frame -> True]