system cannot find the file specified

public string RunExternalExe(string filename, string arguments = null)
       {
           var process = new Process();

           process.StartInfo.FileName = filename;
           if (!string.IsNullOrEmpty(arguments))
           {
               process.StartInfo.Arguments = arguments;
           }

           process.StartInfo.CreateNoWindow = true;
           process.StartInfo.WindowStyle = ProcessWindowStyle.Hidden;
           process.StartInfo.UseShellExecute = false;

           process.StartInfo.RedirectStandardError = true;
           process.StartInfo.RedirectStandardOutput = true;
           var stdOutput = new StringBuilder();
           process.OutputDataReceived += (sender, args) => stdOutput.Append(args.Data);

           string stdError = null;
           try
           {
              // process.Start("rtmpdump.exe");
               System.Diagnostics.Process.Start("rtmpdump", filename);
              // Process.Start(filename);
               process.BeginOutputReadLine();
               stdError = process.StandardError.ReadToEnd();
               process.WaitForExit();
           }
           catch (Exception e)
           {
               throw new Exception("OS error while executing " + Format(filename, arguments) + ": " + e.Message, e);
           }

           if (process.ExitCode == 0 || process.ExitCode == 2)
           {
               string yes = "yes";

               return yes;
           }
           else
           {


               return "no";
           }
       }

       private static string Format(string filename, string arguments)
       {
           return "'" + filename +
               ((string.IsNullOrEmpty(arguments)) ? string.Empty : " " + arguments) +
               "'";
       }